N-methyl amino acids

ABSTRACT

The present invention relates to a compound of formula (I) or (II), processes for preparing them, peptides including them and kits involving them.

The present invention relates to new N-methyl amino acids and theirprecursor oxazolidinones, processes for their preparation and their usein the synthesis of peptides. The invention also includes the use of thenew N-methyl amino acids together with known N-methyl amino acids in akit for synthesising peptides.

BACKGROUND OF THE INVENTION

N-methyl amino acids are secondary metabolites present in a wide varietyof naturally occurring peptides that display a remarkable range ofbiological activities including antibiotic, antiviral, anticancer andantifungal. They are also useful compounds for increasing certainpharmacokinetic parameters such as membrane permeability, proteolyticstability and conformational rigidity. In view of the limitedavailability of N-methyl amino acids, there is a need to prepare suchcompounds and their precursors for use in the solution and solid phasesynthesis of target peptides.

A range of methods have been employed to prepare N-methyl amino acids.These include methods for direct methylation, ¹⁻⁵ reductive amination,⁶⁻¹² alternative methods ¹³⁻¹⁸ and through the generation ofoxazolidinones and their subsequent transformation to the N-methylproduct. ¹⁹⁻²³ In addition, there are strategies involving the use ofimmonium ions in Diels-Alder/retro-Diels-Alder sequences,²⁴ thenucleophilic displacement of triflates,²⁵ the hydroxyamination of chiralenolates²⁶ and the Mitsunobu reaction.²⁷ Some of these methods sufferfrom limitations in the range of amino acids to which they areapplicable, some utilize rather long synthetic sequences and some causeat least partial racemisation of the substrate. We have exploitedoxazolidinone chemistry to generate a range of N-methyl amino acids andtheir precursor oxazolidinones.

The general oxazolidinone route is shown below, where protected aminoacids are cyclized efficiently to oxazolidinones. These oxazolidinonesmay be reductively cleaved by complementary procedures that giveN-methyl amino acids.

We previously reported²⁷ the synthesis, via the carbamates (1) and the5-oxazolidinones (2), of a number of new N-methyl α-amino acids mainlyin the form of their benzyl carbamates (3) and free amino acids (4) asshown below.

a R = CH₂OH b R = CH(OH)CH₃ c R = CH₂PhOH d R = CH₂CH₂COOH (DL) e R = Phf R = CH₂CH₂CH₂CH₂NPhth g R = CH₂SH h R = CH₂CH₂SCH₃ i R = CH₂COOBn j R= CH₂CH₂CONHCbz k R = CH₂CH₂CONH₂ l R = CH₂COOH*(4f) isolated as p-TsOH salt

A focus of that study was the endeavour to demonstrate the generalapplicability of 5-oxazolidinones to the generation of N-methylderivatives of the 20 common natural α-amino acids in the absence of (egglutamic acid and tyrosine) or the minimal presence (eg glutamine andaspartic acid) of side chain protecting groups. This approach wasdesigned to emphasise the efficiency of the oxazolidinone route, itsmildness, as measured by the lack of racemisation of the α-center, andits chemoselectivity. Indeed, the selectivity of the oxazolidinationreaction for the α-amino acid backbone aza and carboxylicfunctionalities often allowed the subsequent manipulation of reactivesidechains.

However in that paper²⁷ there were notable failures in the strategyparticularly in regard to certain difficult α-amino acids, those bearingsidechains such as histidine and tryptophan. We have now successfullysynthesised these outstanding N-methyl targets, together with otherssuch as threonine, serine, cysteine, methionine, asparagine, asparticacid and glutamic acid and their oxazolidinone precursors. As a result,the 5-oxazolidinone route to N-methyl amino acids has now been appliedto the synthesis of all 20 of the common L-α-amino acids and somerelated compounds.

SUMMARY OF THE INVENTION

According to the present invention there is provided a compound offormula I or II:

in which

-   -   R¹ is an N-protecting group or a peptide;    -   R² is CHCH₃OAc or CHR⁵R⁶ in which R⁵ is hydrogen and R⁶ is OAc,        CONH₂, SBn,        CO₂R⁷ or CH₂CO₂R⁷ in which R⁷ is a carboxyl protecting group;        and    -   R³ is CHCH₃OAc,        or CHR⁵R⁶ in which R⁵ is as defined above and R⁶ is OAc, SBn,        CONHTrt,        CO₂R⁷, CHCO₂R⁷, CH₂CH₃ or CH═CH₂ in which R⁷ is as defined        above, R⁸ is a histidine protecting group and R⁹ is a phenol        protecting group;    -   R⁴ is hydrogen or R⁴ is methyl when R³ is OAc;    -   R³ together with R⁴ forms cyclopentyl; or    -   R² and R³ independently represent optionally protected amino        acid side chains selected from:    -   salts, hydrates, solvates, derivatives, tautomers and/or isomers        thereof.

The present invention also provides a process for preparing the compoundof formula I as defined above which comprises reductive cleavage of thecompound of formula II defined above.

The present invention further provides a process for preparing thecompound of formula I or II when

-   -   R¹ is an N-protecting group or a peptide;    -   R² is CHCH₃OAc or CHR⁵R⁶ in which R⁵ is hydrogen and R⁶ is OAc,        CONH₂, SBn,        CO₂R⁷ or CH₂CO₂R⁷ in which R⁷ is a carboxyl protecting group;        and    -   R³ is CHCH₃OAc,        or CHR⁵R⁶ in which R⁵ is as defined above and R⁶ is OAc, SBn,        CONHTrt,        CO₂R⁷, CHCO₂R⁷, CH₂CH₃ or CH═CH₂ in which R⁷ is as defined        above, R⁸ is a histidine protecting group and R⁹ is a phenol        protecting group; and    -   R⁴ is hydrogen or R⁴ is methyl when R³ is OAc; or    -   R³ together with R⁴ forms cyclopentyl, which comprises the steps        of:    -   (a) converting a compound of formula III        in which    -   R² _(a) is CHOHMe or CHR⁵R⁶ _(a) in which R⁵ is as defined above        and R⁶ _(a) is OH, SH, CONH₂,        in which R⁸ is as defined above,        CO₂H or CH₂CONH₂    -   or salts thereof    -   into a compound of formula IV        in which    -   R¹ _(b) is an N-protecting group;    -   R² _(b) is CHOAcMe or CHR⁵R⁶ _(b) in which R⁵ is as defined        above and R⁶ _(b) is OAc, SBn, SMe, CONHR¹ _(b) in which R¹ _(b)        is as defined above,        CO₂H or CH₂CO₂H;    -   (b) oxazolidination of the compound of formula IV to form the        compound of formula II as defined above; and    -   (c) reductive cleavage of the compound of formula II as defined        above to form the compound of formula I as defined above.

Further according to the present invention there is provided use of thecompound of formula I or II defined above in the synthesis of peptides.

Still further according to the present invention there is provided apeptide which includes a compound of formula I or II as defined above,

The invention also extends to a kit for use in synthesising peptideswhich comprises

-   -   (a) at least one compound of formula I or formula II or the        peptide defined above; and    -   (b) optionally at least one other N-methyl amino acid, its        precursor oxazolidinone, an optionally protected amino acid or        protected forms thereof,    -   said compounds, N-methyl amino acids, oxazolidinones and/or        amino acids being held separately.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of this specification it will be clearly understoodthat the word “comprising” means “including but not limited to”, andthat the word “comprises” has a corresponding meaning.

The term “N-protecting group” is used herein in its broadest sense andrefers to any group capable of protecting the amino group of an aminoacid such as those disclosed in Greene, T. W., “Protective Groups inOrganic Synthesis” John Wiley & Sons, New York 1991, pp 315-398 and379-385, the contents of which are incorporated herein by reference.

Preferably the N-protecting group is a carbamate such as,9-fluorenylmethyl carbamate (Fmoc), 2,2,2-trichloroethyl carbamate(Troc), t-butyl carbamate (BOC), allyl carbamate (Alloc),2-trimethylsilylethyl (Teoc) and benzyl carbamate (Cbz or Z), morepreferably Fmoc or Z.

The term “carboxyl-protecting group” is used herein in its broadestsense and refers to any group capable of protecting a carboxyl groupsuch as those disclosed in Green, T. W., “Protective Groups in OrganicSynthesis” John Wiley & Sons, New York 1991, pp 224-276, the contents ofwhich are incorporated herein by reference.

The term “histidine protecting group” is used herein in its broadestsense and refers to any group capable of protecting a histidine groupsuch as carbamates, sulphonyl groups or N-aryl groups for example Z,tosyl, mesyl or 2,4-dinitrophenyl (DNP).

The term “phenol protecting group” is used herein in its broadest senseand refers to any group capable of protecting a phenol group inparticular a tyrosine phenol group for example 2,4-DNP, acyl, alkyl orbenzyl.

The term “amino acid side chain protecting group” is used herein in itsbroadest sense and refers to any suitable known group which is capableof protecting organic functionalities, for example, alcohols, amines,acids, amides or thiols, such as those disclosed in Greene, .W.,“Protective Groups in Organic Synthesis” John Wiley & Sons, New York1991. For example, the carboxyl groups of aspartic acid, glutamic acidand α-aminoadipic acid may be esterified (for example as a C₁-C₆ alkylester), the amino groups of lysine, ornithine and 5-hydroxylysine, maybe converted to carbamates (for example as a C(═O)OC₁-C₆ alkyl orC(═O)OCH₂Ph carbamate) or imides such as phthalimide or succinimide, thehydroxyl groups of 5-hydroxylysine, 4-hydroxyproline, serine, threonine,tyrosine, 3,4-dihydroxyphenylalanine, homoserine, α-methylserine andthyroxine may be converted to ethers (for example a C₁-C₆ alkyl or a(C₁-C₆ alkyl)phenyl ether) or esters (for example a C═OC₁-C₆ alkylester) and the thiol group of cysteine may be converted to thioethers(for example a C₁-C₆ alkyl thioether) or thioesters (for example aC(═O)C₁-C₆ alkyl thioester).

The salts of the compound of Formula I, II or III are preferablypharmaceutically acceptable, but it will be appreciated thatnon-pharmaceutically acceptable salts also fall within the scope of thepresent invention, since these are useful as intermediates in thepreparation of pharmaceutically acceptable salts. Examples ofpharmaceutically acceptable salts include salts of pharmaceuticallyacceptable cations such as sodium, potassium, lithium, calcium,magnesium, ammonium and alkylammonium; acid addition salts ofpharmaceutically acceptable inorganic acids such as hydrochloric,orthophosphoric, sulphuric, phosphoric, nitric, carbonic, boric,sulfamic and hydrobromic acids; or salts of pharmaceutically acceptableorganic acids such as acetic, propionic, butyric, tartaric, maleic,hydroxymaleic, fumaric, citric, lactic, mucic, gluconic, benzoic,succinic, oxalic, phenylacetic, methanesulphonic,trihalomethanesulphonic, toluenesulphonic, benzenesulphonic, salicylic,sulphanilic, aspartic, glutamic, edetic, stearic, palmitic, oleic,lauric, pantothenic, tannic, ascorbic and valeric acids.

In addition, some of the compounds of the present invention may formsolvates with water or common organic solvents. Such solvates areencompassed within the scope of the invention.

By “pharmaceutically acceptable derivative” is meant anypharmaceutically acceptable salt, hydrate, ester, amide, activemetabolite, analogue, residue or any other compound which is notbiologically or otherwise undesirable and induces the desiredpharmacological and/or physiological effect.

The term “tautomer” is used herein in its broadest sense to includecompounds of Formula I or II which are capable of existing in a state ofequilibrium between two isomeric forms. Such compounds may differ in thebond connecting two atoms or groups and the position of these atoms orgroups in the compound.

The term “isomer” is used herein in its broadest sense and includesstructural, geometric and stereo isomers. As the compound of Formula Ior II may have one or more chiral centres, it is capable of existing inenantiomeric forms and/or diastereomeric forms.

Representative examples of compounds of formula I are as follows:

in which

-   -   R¹ is as defined above.

It will be appreciated that these compounds may be present as salts suchas dicyclohexylammonium (DCHA) or tert-butylammonium salts.

Representative examples of compounds of formula II are as follows:

The reductive cleavage may be performed using any suitable knowntechnique, preferably the method described by Freidinger et al²⁸ whichemploys trifluoroacetic acid (TFA) as the acid and triethylsilane(Et₃SiH) as the reductant.

Conversion step (a) results in the protection of the amino group on thecompound of formula III to produce a compound of formula IV. This stepmay be performed using any suitable known technique, such as thosedisclosed in Greene, T. W., “Protective Groups in Organic Synthesis”John Wiley & Sons, New York, 1991.

Step (b) results in cyclisation of the compound of formula IV using anysuitable known technique such as described by Aurelio, L. et al²⁷ usinga formaldehyde source for example paraformaldehyde andparatoluenesulphonic acid (TsOH) in a suitable organic solvent such asbenzene or toluene.

The preferred preparations of compounds (Ia) to (IIn) described aboveare shown in Schemes 1 to 9a below.

The term “peptide” is used herein in its broadest sense and refers to acompound formed by linking amino acids with amide bonds, using the aminogroup of one molecule and the carboxyl group in another. The peptide maybe a dipeptide containing two amino acid residues, a tripeptidecontaining three amino acid residues and so on up to oligopeptides whichcontain relatively short chains of several amino acid residues andlonger polymers which are polypeptides or proteins.

In one embodiment, the peptide is a dipeptide which bears an internalN-methyl amide bond of formula V:

in which

-   -   R¹ and R² are as defined in formula I above, R′ is an optionally        protected amino acid side chain and R is H or a        carboxyl-protecting group.

This is deemed to be useful as it is known in peptide synthesis that thecoupling efficiency of N-methyl residues to the carboxyl terminus of agrowing peptide are not in the vicinity of 99.5% that is required toavoid deletion sequences. By manufacturing dipeptides that already havethe N-methyl amide bond it is thought that potential users will be able,in particular, to program their incorporation into peptide sequenceswhen using automated peptide assembly devices as monomeric species whenin reality the entity incorporated is dimeric. The coupling reaction inthis event will be a standard NH to COOH coupling and so ought to be inthe region of 99.5% yield as the N-methyl bond has already been formedin the dipeptide. Examples of dipeptides of formula V are as follows:MeGly-OR MeAla-OR MeVal-OR MeLeu-OR Z-Gly Z-Gly-MeGly-OR Z-Gly-MeAla-ORZ-Gly-MeVal-OR Z-Gly-MeLeu-OR Z-Ala Z-Ala-MeGly-OR Z-Ala-MeAla-ORZ-Ala-MeVal-OR Z-Ala-MeLeu-OR Z-Val Z-Val-MeGly-OR Z-Val-MeAla-ORZ-Val-MeVal-OR Z-Val-MeLeu-OR Z-Leu Z-Leu-MeGly-OR Z-Leu-MeAla-ORZ-Leu-MeVal-OR Z-Leu-MeLeu-OR Z-Ile Z-Ile-MeGly-OR Z-Ile-MeAla-ORZ-Ile-MeVal-OR Z-Ile-MeLeu-OR Z-Phe Z-Phe-MeGly-OR Z-Phe-MeAla-ORZ-Phe-MeVal-OR Z-Phe-MeLeu-OR Z-Tyr Z-Tyr-MeGly-OR Z-Tyr-MeAla-ORZ-Tyr-MeVal-OR Z-Tyr-MeLeu-OR Z-Ser Z-Ser-MeGly-OR Z-Ser-MeAla-ORZ-Ser-MeVal-OR Z-Ser-MeLeu-OR Z-Thr Z-Thr-MeGly-OR Z-Thr-MeAla-ORZ-Thr-MeVal-OR Z-Thr-MeLeu-OR Z-Cys Z-Cys-MeGly-OR Z-Cys-MeAla-ORZ-Cys-MeVal-OR Z-Cys-MeLeu-OR Z-Met Z-Met-MeGly-OR Z-Met-MeAla-ORZ-Met-MeVal-OR Z-Met-MeLeu-OR Z-Asp Z-Asp-MeGly-OR Z-Asp-MeAla-ORZ-Asp-MeVal-OR Z-Asp-MeLeu-OR Z-Glu Z-Glu-MeGly-OR Z-Glu-MeAla-ORZ-Glu-MeVal-OR Z-Glu-MeLeu-OR Z-Asn Z-Asn-MeGly-OR Z-Asn-MeAla-ORZ-Asn-MeVal-OR Z-Asn-MeLeu-OR Z-Gln Z-Gln-MeGly-OR Z-Gln-MeAla-ORZ-Gln-MeVal-OR Z-Gln-MeLeu-OR Z-Lys Z-Lys-MeGly-OR Z-Lys-MeAla-ORZ-Lys-MeVal-OR Z-Lys-MeLeu-OR Z-Trp Z-Trp-MeGly-OR Z-Trp-MeAla-ORZ-Trp-MeVal-OR Z-Trp-MeLeu-OR Z-His Z-His-MeGly-OR Z-His-MeAla-ORZ-His-MeVal-OR Z-His-MeLeu-OR Z-Arg Z-Arg-MeGly-OR Z-Arg-MeAla-ORZ-Arg-MeVal-OR Z-Arg-MeLeu-OR Z-Pro Z-Pro-MeGly-OR Z-Pro-MeAla-ORZ-Pro-MeVal-OR Z-Pro-MeLeu-OR Z-Orn Z-Orn-MeGly-OR Z-Orn-MeAla-OrZ-Orn-MeVal-OR Z-Orn-MeLeu-OR MeIle-OR MePhe-OR MeTyr-OR MeSer-OR Z-GlyZ-Gly-MeIle-OR Z-Gly-MePhe-OR Z-Gly-MeTyr-OR Z-Gly-MeSer-OR Z-AlaZ-Ala-MeIle-OR Z-Ala-MePhe-OR Z-Ala-MeTyr-OR Z-Ala-MeSer-OR Z-ValZ-Val-MeIle-OR Z-Val-MePhe-OR Z-Val-MeTyr-OR Z-Val-MeSer-OR Z-LeuZ-Leu-MeIle-OR Z-Leu-MePhe-OR Z-Leu-MeTyr-OR Z-Leu-MeSer-OR Z-IleZ-Ile-MeIle-OR Z-Ile-MePhe-OR Z-Ile-MeTyr-OR Z-Ile-MeSer-OR Z-PheZ-Phe-MeIle-OR Z-Phe-MePhe-OR Z-Phe-MeTyr-OR Z-Phe-MeSer-OR Z-TyrZ-Tyr-MeIle-OR Z-Tyr-MePhe-OR Z-Tyr-MeTyr-OR Z-Tyr-MeSer-OR Z-SerZ-Ser-MeIle-OR Z-Ser-MePhe-OR Z-Ser-MeTyr-OR Z-Ser-MeSer-OR Z-ThrZ-Thr-MeIle-OR Z-Thr-MePhe-OR Z-Thr-MeTyr-OR Z-Thr-MeSer-OR Z-CysZ-Cys-MeIle-OR Z-Cys-MePhe-OR Z-Cys-MeTyr-OR Z-Cys-MeSer-OR Z-MetZ-Met-MeIle-OR Z-Met-MePhe-OR Z-Met-MeTyr-OR Z-Met-MeSer-OR Z-AspZ-Asp-MeIle-OR Z-Asp-MePhe-OR Z-Asp-MeTyr-OR Z-Asp-MeSer-OR Z-GluZ-Glu-MeIle-OR Z-Glu-MePhe-OR Z-Glu-MeTyr-OR Z-Glu-MeSer-OR Z-AsnZ-Asn-MeIle-OR Z-Asn-MePhe-OR Z-Asn-MeTyr-OR Z-Asn-MeSer-OR Z-GlnZ-Gln-MeIle-OR Z-Gln-MePhe-OR Z-Gln-MeTyr-OR Z-Gln-MeSer-OR Z-LysZ-Lys-MeIle-OR Z-Lys-MePhe-OR Z-Lys-MeTyr-OR Z-Lys-MeSer-OR Z-TrpZ-Trp-MeIle-OR Z-Trp-MePhe-OR Z-Trp-MeTyr-OR Z-Trp-MeSer-OR Z-HisZ-His-MeIle-OR Z-His-MePhe-OR Z-His-MeTyr-OR Z-His-MeSer-OR Z-ArgZ-Arg-MeIle-OR Z-Arg-MePhe-OR Z-Arg-MeTyr-OR Z-Arg-MeSer-OR Z-ProZ-Pro-MeIle-OR Z-Pro-MePhe-OR Z-Pro-MeTyr-OR Z-Pro-MeSer-OR Z-OrnZ-Orn-MeIle-OR Z-Orn-MePhe-OR Z-Orn-MeTyr-OR Z-Orn-MeSer-OR MeThr-ORMeCys-OR MeMet-OR MeAsp-OR Z-Gly Z-Gly-MeThr-OR Z-Gly-MeCys-ORZ-Gly-MeMet-OR Z-Gly-MeAsp-OR Z-Ala Z-Ala-MeThr-OR Z-Ala-MeCys-ORZ-Ala-MeMet-OR Z-Ala-MeAsp-OR Z-Val Z-Val-MeThr-OR Z-Val-MeCys-ORZ-Val-MeMet-OR Z-Val-MeAsp-OR Z-Leu Z-Leu-MeThr-OR Z-Leu-MeCys-ORZ-Leu-MeMet-OR Z-Leu-MeAsp-OR Z-Ile Z-Ile-MeThr-OR Z-Ile-MeCys-ORZ-Ile-MeMet-OR Z-Ile-MeAsp-OR Z-Phe Z-Phe-MeThr-OR Z-Phe-MeCys-ORZ-Phe-MeMet-OR Z-Phe-MeAsp-OR Z-Tyr Z-Tyr-MeThr-OR Z-Tyr-MeCys-ORZ-Tyr-MeMet-OR Z-Tyr-MeAsp-OR Z-Ser Z-Ser-MeThr-OR Z-Ser-MeCys-ORZ-Ser-MeMet-OR Z-Ser-MeAsp-OR Z-Thr Z-Thr-MeThr-OR Z-Thr-MeCys-ORZ-Thr-MeMet-OR Z-Thr-MeAsp-OR Z-Cys Z-Cys-MeThr-OR Z-Cys-MeCys-ORZ-Cys-MeMet-OR Z-Cys-MeAsp-OR Z-Met Z-Met-MeThr-OR Z-Met-MeCys-ORZ-Met-MeMet-OR Z-Met-MeAsp-OR Z-Asp Z-Asp-MeThr-OR Z-Asp-MeCys-ORZ-Asp-MeMet-OR Z-Asp-MeAsp-OR Z-Glu Z-Glu-MeThr-OR Z-Glu-MeCys-ORZ-Glu-MeMet-OR Z-Glu-MeAsp-OR Z-Asn Z-Asn-MeThr-OR Z-Asn-MeCys-ORZ-Asn-MeMet-OR Z-Asn-MeAsp-OR Z-Gln Z-Gln-MeThr-OR Z-Gln-MeCys-ORZ-Gln-MeMet-OR Z-Gln-MeAsp-OR Z-Lys Z-Lys-MeThr-OR Z-Lys-MeCys-ORZ-Lys-MeMet-OR Z-Lys-MeAsp-OR Z-Trp Z-Trp-MeThr-OR Z-Trp-MeCys-ORZ-Trp-MeMet-OR Z-Trp-MeAsp-OR Z-His Z-His-MeThr-OR Z-His-MeCys-ORZ-His-MeMet-OR Z-His-MeAsp-OR Z-Arg Z-Arg-MeThr-OR Z-Arg-MeCys-ORZ-Arg-MeMet-OR Z-Arg-MeAsp-OR Z-Pro Z-Pro-MeThr-OR Z-Pro-MeCys-ORZ-Pro-MeMet-OR Z-Pro-MeAsp-OR Z-Orn Z-Orn-MeThr-OR Z-Orn-MeCys-ORZ-Orn-MeMet-OR Z-Orn-MeAsp-OR MeGlu-OR MeAsn-OR MeGln-OR MeLys-OR Z-GlyZ-Gly-MeGlu-OR Z-Gly-MeAsn-OR Z-Gly-MeGln-OR Z-Gly-MeLys-OR Z-AlaZ-Ala-MeGlu-OR Z-Ala-MeAsn-OR Z-Ala-MeGln-OR Z-Ala-MeLys-OR Z-ValZ-Val-MeGlu-OR Z-Val-MeAsn-OR Z-Val-MeGln-OR Z-Val-MeLys-OR Z-LeuZ-Leu-MeGlu-OR Z-Leu-MeAsn-OR Z-Leu-MeGln-OR Z-Leu-MeLys-OR Z-IleZ-Ile-MeGlu-OR Z-Ile-MeAsn-OR Z-Ile-MeGln-OR Z-Ile-MeLys-OR Z-PheZ-Phe-MeGlu-OR Z-Phe-MeAsn-OR Z-Phe-MeGln-OR Z-Phe-MeLys-OR Z-TyrZ-Tyr-MeGlu-OR Z-Tyr-MeAsn-OR Z-Tyr-MeGln-OR Z-Tyr-MeLys-OR Z-SerZ-Ser-MeGlu-OR Z-Ser-MeAsn-OR Z-Ser-MeGln-OR Z-Ser-MeLys-OR Z-ThrZ-Thr-MeGlu-OR Z-Thr-MeAsn-OR Z-Thr-MeGln-OR Z-Thr-MeLys-OR Z-CysZ-Cys-MeGlu-OR Z-Cys-MeAsn-OR Z-Cys-MeGln-OR Z-Cys-MeLys-OR Z-MetZ-Met-MeGlu-OR Z-Met-MeAsn-OR Z-Met-MeGln-OR Z-Met-MeLys-OR Z-AspZ-Asp-MeGlu-OR Z-Asp-MeAsn-OR Z-Asp-MeGln-OR Z-Asp-MeLys-OR Z-GluZ-Glu-MeGlu-OR Z-Glu-MeAsn-OR Z-Glu-MeGln-OR Z-Glu-MeLys-OR Z-AsnZ-Asn-MeGlu-OR Z-Asn-MeAsn-OR Z-Asn-MeGln-OR Z-Asn-MeLys-OR Z-GlnZ-Gln-MeGlu-OR Z-Gln-MeAsn-OR Z-Gln-MeGln-OR Z-Gln-MeLys-OR Z-LysZ-Lys-MeGlu-OR Z-Lys-MeAsn-OR Z-Lys-MeGln-OR Z-Lys-MeLys-OR Z-TrpZ-Trp-MeGlu-OR Z-Trp-MeAsn-OR Z-Trp-MeGln-OR Z-Trp-MeLys-OR Z-HisZ-His-MeGlu-OR Z-His-MeAsn-OR Z-His-MeGln-OR Z-His-MeLys-OR Z-ArgZ-Arg-MeGlu-OR Z-Arg-MeAsn-OR Z-Arg-MeGln-OR Z-Arg-MeLys-OR Z-ProZ-Pro-MeGlu-OR Z-Pro-MeAsn-OR Z-Pro-MeGln-OR Z-Pro-MeLys-OR Z-OrnZ-Orn-MeGlu-OR Z-Orn-MeAsn-OR Z-Orn-MeGln-OR Z-Orn-MeLys-OR MeTrp-ORMeHis-OR MeArg-OR MeOrn-OR Z-Gly Z-Gly-MeTrp-OR Z-Gly-MeHis-ORZ-Gly-MeArg-OR Z-Gly-MeOrn-OR Z-Ala Z-Ala-MeTrp-OR Z-Ala-MeHis-ORZ-Ala-MeArg-OR Z-Ala-MeOrn-OR Z-Val Z-Val-MeTrp-OR Z-Val-MeHis-ORZ-Val-MeArg-OR Z-Val-MeOrn-OR Z-Leu Z-Leu-MeTrp-OR Z-Leu-MeHis-ORZ-Leu-MeArg-OR Z-Leu-MeOrn-OR Z-Ile Z-Ile-MeTrp-OR Z-Ile-MeHis-ORZ-Ile-MeArg-OR Z-Ile-MeOrn-OR Z-Phe Z-Phe-MeTrp-OR Z-Phe-MeHis-ORZ-Phe-MeArg-OR Z-Phe-MeOrn-OR Z-Tyr Z-Tyr-MeTrp-OR Z-Tyr-MeHis-ORZ-Tyr-MeArg-OR Z-Tyr-MeOrn-OR Z-Ser Z-Ser-MeTrp-OR Z-Ser-MeHis-ORZ-Ser-MeArg-OR Z-Ser-MeOrn-OR Z-Thr Z-Thr-MeTrp-OR Z-Thr-MeHis-ORZ-Thr-MeArg-OR Z-Thr-MeOrn-OR Z-Cys Z-Cys-MeTrp-OR Z-Cys-MeHis-ORZ-Cys-MeArg-OR Z-Cys-MeOrn-OR Z-Met Z-Met-MeTrp-OR Z-Met-MeHis-ORZ-Met-MeArg-OR Z-Met-MeOrn-OR Z-Asp Z-Asp-MeTrp-OR Z-Asp-MeHis-ORZ-Asp-MeArg-OR Z-Asp-MeOrn-OR Z-Glu Z-Glu-MeTrp-OR Z-Glu-MeHis-ORZ-Glu-MeArg-OR Z-Glu-MeOrn-OR Z-Asn Z-Asn-MeTrp-OR Z-Asn-MeHis-ORZ-Asn-MeArg-OR Z-Asn-MeOrn-OR Z-Gln Z-Gln-MeTrp-OR Z-Gln-MeHis-ORZ-Gln-MeArg-OR Z-Gln-MeOrn-OR Z-Lys Z-Lys-MeTrp-OR Z-Lys-MeHis-ORZ-Lys-MeArg-OR Z-Lys-MeOrn-OR Z-Trp Z-Trp-MeTrp-OR Z-Trp-MeHis-ORZ-Trp-MeArg-OR Z-Trp-MeOrn-OR Z-His Z-His-MeTrp-OR Z-His-MeHis-ORZ-His-MeArg-OR Z-His-MeOrn-OR Z-Arg Z-Arg-MeTrp-OR Z-Arg-MeHis-ORZ-Arg-MeArg-OR Z-Arg-MeOrn-OR Z-Pro Z-Pro-MeTrp-OR Z-Pro-MeHis-ORZ-Pro-MeArg-OR Z-Pro-MeOrn-OR Z-Orn Z-Orn-MeTrp-OR Z-Orn-MeHis-ORZ-Orn-MeArg-OR Z-Orn-MeOrn-OR MeGly-OR MeAla-OR MeVal-OR MeLeu-ORFmoc-Gly Fmoc-Gly-MeGly-OR Fmoc-Gly-MeAla-OR Fmoc-Gly-MeVal-ORFmoc-Gly-MeLeu-OR Fmoc-Ala Fmoc-Ala-MeGly-OR Fmoc-Ala-MeAla-ORFmoc-Ala-MeVal-OR Fmoc-Ala-MeLeu-OR Fmoc-Val Fmoc-Val-MeGly-ORFmoc-Val-MeAla-OR Fmoc-Val-MeVal-OR Fmoc-Val-MeLeu-OR Fmoc-LeuFmoc-Leu-MeGly-OR Fmoc-Leu-MeAla-OR Fmoc-Leu-MeVal-OR Fmoc-Leu-MeLeu-ORFmoc-Ile Fmoc-Ile-MeGly-OR Fmoc-Ile-MeAla-OR Fmoc-Ile-MeVal-ORFmoc-Ile-MeLeu-OR Fmoc-Phe Fmoc-Phe-MeGly-OR Fmoc-Phe-MeAla-ORFmoc-Phe-MeVal-OR Fmoc-Phe-MeLeu-OR Fmoc-Tyr Fmoc-Tyr-MeGly-ORFmoc-Tyr-MeAla-OR Fmoc-Tyr-MeVal-OR Fmoc-Tyr-MeLeu-OR Fmoc-SerFmoc-Ser-MeGly-OR Fmoc-Ser-MeAla-OR Fmoc-Ser-MeVal-OR Fmoc-Ser-MeLeu-ORFmoc-Thr Fmoc-Thr-MeGly-OR Fmoc-Thr-MeAla-OR Fmoc-Thr-MeVal-ORFmoc-Thr-MeLeu-OR Fmoc-Cys Fmoc-Cys-MeGly-OR Fmoc-Cys-MeAla-ORFmoc-Cys-MeVal-OR Fmoc-Cys-MeLeu-OR Fmoc-Met Fmoc-Met-MeGly-ORFmoc-Met-MeAla-OR Fmoc-Met-MeVal-OR Fmoc-Met-MeLeu-OR Fmoc-AspFmoc-Asp-MeGly-OR Fmoc-Asp-MeAla-OR Fmoc-Asp-MeVal-OR Fmoc-Asp-MeLeu-ORFmoc-Glu Fmoc-Glu-MeGly-OR Fmoc-Glu-MeAla-OR Fmoc-Glu-MeVal-ORFmoc-Glu-MeLeu-OR Fmoc-Asn Fmoc-Asn-MeGly-OR Fmoc-Asn-MeAla-ORFmoc-Asn-MeVal-OR Fmoc-Asn-MeLeu-OR Fmoc-Gln Fmoc-Gln-MeGly-ORFmoc-Gln-MeAla-OR Fmoc-Gln-MeVal-OR Fmoc-Gln-MeLeu-OR Fmoc-LysFmoc-Lys-MeGly-OR Fmoc-Lys-MeAla-OR Fmoc-Lys-MeVal-OR Fmoc-Lys-MeLeu-ORFmoc-Trp Fmoc-Trp-MeGly-OR Fmoc-Trp-MeAla-OR Fmoc-Trp-MeVal-ORFmoc-Trp-MeLeu-OR Fmoc-His Fmoc-His-MeGly-OR Fmoc-His-MeAla-ORFmoc-His-MeVal-OR Fmoc-His-MeLeu-OR Fmoc-Arg Fmoc-Arg-MeGly-ORFmoc-Arg-MeAla-OR Fmoc-Arg-MeVal-OR Fmoc-Arg-MeLeu-OR Fmoc-ProFmoc-Pro-MeGly-OR Fmoc-Pro-MeAla-OR Fmoc-Pro-MeVal-OR Fmoc-Pro-MeLeu-ORFmoc-Orn Fmoc-Orn-MeGly-OR Fmoc-Orn-MeAla-OR Fmoc-Orn-MeVal-ORFmoc-Orn-MeLeu-OR MeIle-OR MePhe-OR MeTyr-OR Fmoc-Gly Fmoc-Gly-MeIle-ORFmoc-Gly-MePhe-OR Fmoc-Gly-MeTyr-OR Fmoc-Ala Fmoc-Ala-MeIle-ORFmoc-Ala-MePhe-OR Fmoc-Ala-MeTyr-OR Fmoc-Val Fmoc-Val-MeIle-ORFmoc-Val-MePhe-OR Fmoc-Val-MeTyr-OR Fmoc-Leu Fmoc-Leu-MeIle-ORFmoc-Leu-MePhe-OR Fmoc-Leu-MeTyr-OR Fmoc-Ile Fmoc-Ile-MeIle-ORFmoc-Ile-MePhe-OR Fmoc-Ile-MeTyr-OR Fmoc-Phe Fmoc-Phe-MeIle-ORFmoc-Phe-MePhe-OR Fmoc-Phe-MeTyr-OR Fmoc-Tyr Fmoc-Tyr-MeIle-ORFmoc-Tyr-MePhe-OR Fmoc-Tyr-MeTyr-OR Fmoc-Ser Fmoc-Ser-MeIle-ORFmoc-Ser-MePhe-OR Fmoc-Ser-MeTyr-OR Fmoc-Thr Fmoc-Thr-MeIle-ORFmoc-Thr-MePhe-OR Fmoc-Thr-MeTyr-OR Fmoc-Cys Fmoc-Cys-MeIle-ORFmoc-Cys-MePhe-OR Fmoc-Cys-MeTyr-OR Fmoc-Met Fmoc-Met-MeIle-ORFmoc-Met-MePhe-OR Fmoc-Met-MeTyr-OR Fmoc-Asp Fmoc-Asp-MeIle-ORFmoc-Asp-MePhe-OR Fmoc-Asp-MeTyr-OR Fmoc-Glu Fmoc-Glu-MeIle-ORFmoc-Glu-MePhe-OR Fmoc-Glu-MeTyr-OR Fmoc-Asn Fmoc-Asn-MeIle-ORFmoc-Asn-MePhe-OR Fmoc-Asn-MeTyr-OR Fmoc-Gln Fmoc-Gln-MeIle-ORFmoc-Gln-MePhe-OR Fmoc-Gln-MeTyr-OR Fmoc-Lys Fmoc-Lys-MeIle-ORFmoc-Lys-MePhe-OR Fmoc-Lys-MeTyr-OR Fmoc-Trp Fmoc-Trp-MeIle-ORFmoc-Trp-MePhe-OR Fmoc-Trp-MeTyr-OR Fmoc-His Fmoc-His-MeIle-ORFmoc-His-MePhe-OR Fmoc-His-MeTyr-OR Fmoc-Arg Fmoc-Arg-MeIle-ORFmoc-Arg-MePhe-OR Fmoc-Arg-MeTyr-OR Fmoc-Pro Fmoc-Pro-MeIle-ORFmoc-Pro-MePhe-OR Fmoc-Pro-MeTyr-OR Fmoc-Orn Fmoc-Orn-MeIle-ORFmoc-Orn-MePhe-OR Fmoc-Orn-MeTyr-OR MeSer-OR MeThr-OR MeCys-OR MeMet-ORFmoc-Gly Fmoc-Gly-MeSer-OR Fmoc-Gly-MeThr-OR Fmoc-Gly-MeCys-ORFmoc-Gly-MeMet-OR Fmoc-Ala Fmoc-Ala-MeSer-OR Fmoc-Ala-MeThr-ORFmoc-Ala-MeCys-OR Fmoc-Ala-MeMet-OR Fmoc-Val Fmoc-Val-MeSer-ORFmoc-Val-MeThr-OR Fmoc-Val-MeCys-OR Fmoc-Val-MeMet-OR Fmoc-LeuFmoc-Leu-MeSer-OR Fmoc-Leu-MeThr-OR Fmoc-Leu-MeCys-OR Fmoc-Leu-MeMet-ORFmoc-Ile Fmoc-Ile-MeSer-OR Fmoc-Ile-MeThr-OR Fmoc-Ile-MeCys-ORFmoc-Ile-MeMet-OR Fmoc-Phe Fmoc-Phe-MeSer-OR Fmoc-Phe-MeThr-ORFmoc-Phe-MeCys-OR Fmoc-Phe-MeMet-OR Fmoc-Tyr Fmoc-Tyr-MeSer-ORFmoc-Tyr-MeThr-OR Fmoc-Tyr-MeCys-OR Fmoc-Tyr-MeMet-OR Fmoc-SerFmoc-Ser-MeSer-OR Fmoc-Ser-MeThr-OR Fmoc-Ser-MeCys-OR Fmoc-Ser-MeMet-ORFmoc-Thr Fmoc-Thr-MeSer-OR Fmoc-Thr-MeThr-OR Fmoc-Thr-MeCys-ORFmoc-Thr-MeMet-OR Fmoc-Cys Fmoc-Cys-MeSer-OR Fmoc-Cys-MeThr-ORFmoc-Cys-MeCys-OR Fmoc-Cys-MeMet-OR Fmoc-Met Fmoc-Met-MeSer-ORFmoc-Met-MeThr-OR Fmoc-Met-MeCys-OR Fmoc-Met-MeMet-OR Fmoc-AspFmoc-Asp-MeSer-OR Fmoc-Asp-MeThr-OR Fmoc-Asp-MeCys-OR Fmoc-Asp-MeMet-ORFmoc-Glu Fmoc-Glu-MeSer-OR Fmoc-Glu-MeThr-OR Fmoc-Glu-MeCys-ORFmoc-Glu-MeMet-OR Fmoc-Asn Fmoc-Asn-MeSer-OR Fmoc-Asn-MeThr-ORFmoc-Asn-MeCys-OR Fmoc-Asn-MeMet-OR Fmoc-Gln Fmoc-Gln-MeSer-ORFmoc-Gln-MeThr-OR Fmoc-Gln-MeCys-OR Fmoc-Gln-MeMet-OR Fmoc-LysFmoc-Lys-MeSer-OR Fmoc-Lys-MeThr-OR Fmoc-Lys-MeCys-OR Fmoc-Lys-MeMet-ORFmoc-Trp Fmoc-Trp-MeSer-OR Fmoc-Trp-MeThr-OR Fmoc-Trp-MeCys-ORFmoc-Trp-MeMet-OR Fmoc-His Fmoc-His-MeSer-OR Fmoc-His-MeThr-ORFmoc-His-MeCys-OR Fmoc-His-MeMet-OR Fmoc-Arg Fmoc-Arg-MeSer-ORFmoc-Arg-MeThr-OR Fmoc-Arg-MeCys-OR Fmoc-Arg-MeMet-OR Fmoc-ProFmoc-Pro-MeSer-OR Fmoc-Pro-MeThr-OR Fmoc-Pro-MeCys-OR Fmoc-Pro-MeMet-ORFmoc-Orn Fmoc-Orn-MeSer-OR Fmoc-Orn-MeThr-OR Fmoc-Orn-MeCys-ORFmoc-Orn-MeMet-OR MeAsp-OR MeGlu-OR MeAsn-OR Fmoc-Gly Fmoc-Gly-MeAsp-ORFmoc-Gly-MeGlu-OR Fmoc-Gly-MeAsn-OR Fmoc-Ala Fmoc-Ala-MeAsp-ORFmoc-Ala-MeGlu-OR Fmoc-Ala-MeAsn-OR Fmoc-Val Fmoc-Val-MeAsp-ORFmoc-Val-MeGlu-OR Fmoc-Val-MeAsn-OR Fmoc-Leu Fmoc-Leu-MeAsp-ORFmoc-Leu-MeGlu-OR Fmoc-Leu-MeAsn-OR Fmoc-Ile Fmoc-Ile-MeAsp-ORFmoc-Ile-MeGlu-OR Fmoc-Ile-MeAsn-OR Fmoc-Phe Fmoc-Phe-MeAsp-ORFmoc-Phe-MeGlu-OR Fmoc-Phe-MeAsn-OR Fmoc-Tyr Fmoc-Tyr-MeAsp-ORFmoc-Tyr-MeGlu-OR Fmoc-Tyr-MeAsn-OR Fmoc-Ser Fmoc-Ser-MeAsp-ORFmoc-Ser-MeGlu-OR Fmoc-Ser-MeAsn-OR Fmoc-Thr Fmoc-Thr-MeAsp-ORFmoc-Thr-MeGlu-OR Fmoc-Thr-MeAsn-OR Fmoc-Cys Fmoc-Cys-MeAsp-ORFmoc-Cys-MeGlu-OR Fmoc-Cys-MeAsn-OR Fmoc-Met Fmoc-Met-MeAsp-ORFmoc-Met-MeGlu-OR Fmoc-Met-MeAsn-OR Fmoc-Asp Fmoc-Asp-MeAsp-ORFmoc-Asp-MeGlu-OR Fmoc-Asp-MeAsn-OR Fmoc-Glu Fmoc-Glu-MeAsp-ORFmoc-Glu-MeGlu-OR Fmoc-Glu-MeAsn-OR Fmoc-Asn Fmoc-Asn-MeAsp-ORFmoc-Asn-MeGlu-OR Fmoc-Asn-MeAsn-OR Fmoc-Gln Fmoc-Gln-MeAsp-ORFmoc-Gln-MeGlu-OR Fmoc-Gln-MeAsn-OR Fmoc-Lys Fmoc-Lys-MeAsp-ORFmoc-Lys-MeGlu-OR Fmoc-Lys-MeAsn-OR Fmoc-Trp Fmoc-Trp-MeAsp-ORFmoc-Trp-MeGlu-OR Fmoc-Trp-MeAsn-OR Fmoc-His Fmoc-His-MeAsp-ORFmoc-His-MeGlu-OR Fmoc-His-MeAsn-OR Fmoc-Arg Fmoc-Arg-MeAsp-ORFmoc-Arg-MeGlu-OR Fmoc-Arg-MeAsn-OR Fmoc-Pro Fmoc-Pro-MeAsp-ORFmoc-Pro-MeGlu-OR Fmoc-Pro-MeAsn-OR Fmoc-Orn Fmoc-Orn-MeAsp-ORFmoc-Orn-MeGlu-OR Fmoc-Orn-MeAsn-OR MeGln-OR MeLys-OR MeTrp-OR Fmoc-GlyFmoc-Gly-MeGln-OR Fmoc-Gly-MeLys-OR Fmoc-Gly-MeTrp-OR Fmoc-AlaFmoc-Ala-MeGln-OR Fmoc-Ala-MeLys-OR Fmoc-Ala-MeTrp-OR Fmoc-ValFmoc-Val-MeGln-OR Fmoc-Val-MeLys-OR Fmoc-Val-MeTrp-OR Fmoc-LeuFmoc-Leu-MeGln-OR Fmoc-Leu-MeLys-OR Fmoc-Leu-MeTrp-OR Fmoc-IleFmoc-Ile-MeGln-OR Fmoc-Ile-MeLys-OR Fmoc-Ile-MeTrp-OR Fmoc-PheFmoc-Phe-MeGln-OR Fmoc-Phe-MeLys-OR Fmoc-Phe-MeTrp-OR Fmoc-TyrFmoc-Tyr-MeGln-OR Fmoc-Tyr-MeLys-OR Fmoc-Tyr-MeTrp-OR Fmoc-SerFmoc-Ser-MeGln-OR Fmoc-Ser-MeLys-OR Fmoc-Ser-MeTrp-OR Fmoc-ThrFmoc-Thr-MeGln-OR Fmoc-Thr-MeLys-OR Fmoc-Thr-MeTrp-OR Fmoc-CysFmoc-Cys-MeGln-OR Fmoc-Cys-MeLys-OR Fmoc-Cys-MeTrp-OR Fmoc-MetFmoc-Met-MeGln-OR Fmoc-Met-MeLys-OR Fmoc-Met-MeTrp-OR Fmoc-AspFmoc-Asp-MeGln-OR Fmoc-Asp-MeLys-OR Fmoc-Asp-MeTrp-OR Fmoc-GluFmoc-Glu-MeGln-OR Fmoc-Glu-MeLys-OR Fmoc-Glu-MeTrp-OR Fmoc-AsnFmoc-Asn-MeGln-OR Fmoc-Asn-MeLys-OR Fmoc-Asn-MeTrp-OR Fmoc-GlnFmoc-Gln-MeGln-OR Fmoc-Gln-MeLys-OR Fmoc-Gln-MeTrp-OR Fmoc-LysFmoc-Lys-MeGln-OR Fmoc-Lys-MeLys-OR Fmoc-Lys-MeTrp-OR Fmoc-TrpFmoc-Trp-MeGln-OR Fmoc-Trp-MeLys-OR Fmoc-Trp-MeTrp-OR Fmoc-HisFmoc-His-MeGln-OR Fmoc-His-MeLys-OR Fmoc-His-MeTrp-OR Fmoc-ArgFmoc-Arg-MeGln-OR Fmoc-Arg-MeLys-OR Fmoc-Arg-MeTrp-OR Fmoc-ProFmoc-Pro-MeGln-OR Fmoc-Pro-MeLys-OR Fmoc-Pro-MeTrp-OR Fmoc-OrnFmoc-Orn-MeGln-OR Fmoc-Orn-MeLys-OR Fmoc-Orn-MeTrp-OR MeHis-OR MeArg-ORMeOrn-OR Fmoc-Gly Fmoc-Gly-MeHis-OR Fmoc-Gly-MeArg-OR Fmoc-Gly-MeOrn-ORFmoc-Ala Fmoc-Ala-MeHis-OR Fmoc-Ala-MeArg-OR Fmoc-Ala-MeOrn-OR Fmoc-ValFmoc-Val-MeHis-OR Fmoc-Val-MeArg-OR Fmoc-Val-MeOrn-OR Fmoc-LeuFmoc-Leu-MeHis-OR Fmoc-Leu-MeArg-OR Fmoc-Leu-MeOrn-OR Fmoc-IleFmoc-Ile-MeHis-OR Fmoc-Ile-MeArg-OR Fmoc-Ile-MeOrn-OR Fmoc-PheFmoc-Phe-MeHis-OR Fmoc-Phe-MeArg-OR Fmoc-Phe-MeOrn-OR Fmoc-TyrFmoc-Tyr-MeHis-OR Fmoc-Tyr-MeArg-OR Fmoc-Tyr-MeOrn-OR Fmoc-SerFmoc-Ser-MeHis-OR Fmoc-Ser-MeArg-OR Fmoc-Ser-MeOrn-OR Fmoc-ThrFmoc-Thr-MeHis-OR Fmoc-Thr-MeArg-OR Fmoc-Thr-MeOrn-OR Fmoc-CysFmoc-Cys-MeHis-OR Fmoc-Cys-MeArg-OR Fmoc-Cys-MeOrn-OR Fmoc-MetFmoc-Met-MeHis-OR Fmoc-Met-MeArg-OR Fmoc-Met-MeOrn-OR Fmoc-AspFmoc-Asp-MeHis-OR Fmoc-Asp-MeArg-OR Fmoc-Asp-MeOrn-OR Fmoc-GluFmoc-Glu-MeHis-OR Fmoc-Glu-MeArg-OR Fmoc-Glu-MeOrn-OR Fmoc-AsnFmoc-Asn-MeHis-OR Fmoc-Asn-MeArg-OR Fmoc-Asn-MeOrn-OR Fmoc-GlnFmoc-Gln-MeHis-OR Fmoc-Gln-MeArg-OR Fmoc-Gln-MeOrn-OR Fmoc-LysFmoc-Lys-MeHis-OR Fmoc-Lys-MeArg-OR Fmoc-Lys-MeOrn-OR Fmoc-Trp Fmoc-TrpMeHis-OR Fmoc-Trp-MeArg-OR Fmoc-Trp-MeOrn-OR Fmoc-His Fmoc-His-MeHis-ORFmoc-His-MeArg-OR Fmoc-His-MeOrn-OR Fmoc-Arg Fmoc-Arg-MeHis-ORFmoc-Arg-MeArg-OR Fmoc-Arg-MeOrn-OR Fmoc-Pro Fmoc-Pro-MeHis-ORFmoc-Pro-MeArg-OR Fmoc-Pro-MeOrn-OR Fmoc-Orn Fmoc-Orn-MeHis-ORFmoc-Orn-MeArg-OR Fmoc-Orn-MeOrn-OR MeGly-OR MeAla-OR MeVal-OR MeLeu-ORBoc-Gly Boc-Gly-MeGly-OR Boc-Gly-MeAla-OR Boc-Gly-MeVal-ORBoc-Gly-MeLeu-OR Boc-Ala Boc-Ala-MeGly-OR Boc-Ala-MeAla-ORBoc-Ala-MeVal-OR Boc-Ala-MeLeu-OR Boc-Val Boc-Val-MeGly-ORBoc-Val-MeAla-OR Boc-Val-MeVal-OR Boc-Val-MeLeu-OR Boc-LeuBoc-Leu-MeGly-OR Boc-Leu-MeAla-OR Boc-Leu-MeVal-OR Boc-Leu-MeLeu-ORBoc-Ile Boc-Ile-MeGly-OR Boc-Ile-MeAla-OR Boc-Ile-MeVal-ORBoc-Ile-MeLeu-OR Boc-Phe Boc-Phe-MeGly-OR Boc-Phe-MeAla-ORBoc-Phe-MeVal-OR Boc-Phe-MeLeu-OR Boc-Tyr Boc-Tyr-MeGly-ORBoc-Tyr-MeAla-OR Boc-Tyr-MeVal-OR Boc-Tyr-MeLeu-OR Boc-SerBoc-Ser-MeGly-OR Boc-Ser-MeAla-OR Boc-Ser-MeVal-OR Boc-Ser-MeLeu-ORBoc-Thr Boc-Thr-MeGly-OR Boc-Thr-MeAla-OR Boc-Thr-MeVal-ORBoc-Thr-MeLeu-OR Boc-Cys Boc-Cys-MeGly-OR Boc-Cys-MeAla-ORBoc-Cys-MeVal-OR Boc-Cys-MeLeu-OR Boc-Met Boc-Met-MeGly-ORBoc-Met-MeAla-OR Boc-Met-MeVal-OR Boc-Met-MeLeu-OR Boc-AspBoc-Asp-MeGly-OR Boc-Asp-MeAla-OR Boc-Asp-MeVal-OR Boc-Asp-MeLeu-ORBoc-Glu Boc-Glu-MeGly-OR Boc-Glu-MeAla-OR Boc-Glu-MeVal-ORBoc-Glu-MeLeu-OR Boc-Asn Boc-Asn-MeGly-OR Boc-Asn-MeAla-ORBoc-Asn-MeVal-OR Boc-Asn-MeLeu-OR Boc-Gln Boc-Gln-MeGly-ORBoc-Gln-MeAla-OR Boc-Gln-MeVal-OR Boc-Gln-MeLeu-OR Boc-LysBoc-Lys-MeGly-OR Boc-Lys-MeAla-OR Boc-Lys-MeVal-OR Boc-Lys-MeLeu-ORBoc-Trp Boc-Trp-MeGly-OR Boc-Trp-MeAla-OR Boc-Trp-MeVal-ORBoc-Trp-MeLeu-OR Boc-His Boc-His-MeGly-OR Boc-His-MeAla-ORBoc-His-MeVal-OR Boc-His-MeLeu-OR Boc-Arg Boc-Arg-MeGly-ORBoc-Arg-MeAla-OR Boc-Arg-MeVal-OR Boc-Arg-MeLeu-OR Boc-ProBoc-Pro-MeGly-OR Boc-Pro-MeAla-OR Boc-Pro-MeVal-OR Boc-Pro-MeLeu-ORBoc-Orn Boc-Orn-MeGly-OR Boc-Orn-MeAla-OR Boc-Orn-MeVal-ORBoc-Orn-MeLeu-OR MeIle-OR MePhe-OR MeTyr-OR Boc-Gly Boc-Gly-MeIle-ORBoc-Gly-MePhe-OR Boc-Gly-MeTyr-OR Boc-Ala Boc-Ala-MeIle-ORBoc-Ala-MePhe-OR Boc-Ala-MeTyr-OR Boc-Val Boc-Val-MeIle-ORBoc-Val-MePhe-OR Boc-Val-MeTyr-OR Boc-Leu Boc-Leu-MeIle-ORBoc-Leu-MePhe-OR Boc-Leu-MeTyr-OR Boc-Ile Boc-Ile-MeIle-ORBoc-Ile-MePhe-OR Boc-Ile-MeTyr-OR Boc-Phe Boc-Phe-MeIle-ORBoc-Phe-MePhe-OR Boc-Phe-MeTyr-OR Boc-Tyr Boc-Tyr-MeIle-ORBoc-Tyr-MePhe-OR Boc-Tyr-MeTyr-OR Boc-Ser Boc-Ser-MeIle-ORBoc-Ser-MePhe-OR Boc-Ser-MeTyr-OR Boc-Thr Boc-Thr-MeIle-ORBoc-Thr-MePhe-OR Boc-Thr-MeTyr-OR Boc-Cys Boc-Cys-MeIle-ORBoc-Cys-MePhe-OR Boc-Cys-MeTyr-OR Boc-Met Boc-Met-MeIle-ORBoc-Met-MePhe-OR Boc-Met-MeTyr-OR Boc-Asp Boc-Asp-MeIle-ORBoc-Asp-MePhe-OR Boc-Asp-MeTyr-OR Boc-Glu Boc-Glu-MeIle-ORBoc-Glu-MePhe-OR Boc-Glu-MeTyr-OR Boc-Asn Boc-Asn-MeIle-ORBoc-Asn-MePhe-OR Boc-Asn-MeTyr-OR Boc-Gln Boc-Gln-MeIle-ORBoc-Gln-MePhe-OR Boc-Gln-MeTyr-OR Boc-Lys Boc-Lys-MeIle-ORBoc-Lys-MePhe-OR Boc-Lys-MeTyr-OR Boc-Trp Boc-Trp-MeIle-ORBoc-Trp-MePhe-OR Boc-Trp-MeTyr-OR Boc-His Boc-His-MeIle-ORBoc-His-MePhe-OR Boc-His-MeTyr-OR Boc-Arg Boc-Arg-MeIle-ORBoc-Arg-MePhe-OR Boc-Arg-MeTyr-OR Boc-Pro Boc-Pro-MeIle-ORBoc-Pro-MePhe-OR Boc-Pro-MeTyr-OR Boc-Orn Boc-Orn-MeIle-ORBoc-Orn-MePhe-OR Boc-Orn-MeTyr-OR MeSer-OR MeThr-OR MeCys-OR MeMet-ORBoc-Gly Boc-Gly-MeSer-OR Boc-Gly-MeThr-OR Boc-Gly-MeCys-ORBoc-Gly-MeMet-OR Boc-Ala Boc-Ala-MeSer-OR Boc-Ala-MeThr-ORBoc-Ala-MeCys-OR Boc-Ala-MeMet-OR Boc-Val Boc-Val-MeSer-ORBoc-Val-MeThr-OR Boc-Val-MeCys-OR Boc-Val-MeMet-OR Boc-LeuBoc-Leu-MeSer-OR Boc-Leu-MeThr-OR Boc-Leu-MeCys-OR Boc-Leu-MeMet-ORBoc-Ile Boc-Ile-MeSer-OR Boc-Ile-MeThr-OR Boc-Ile-MeCys-ORBoc-Ile-MeMet-OR Boc-Phe Boc-Phe-MeSer-OR Boc-Phe-MeThr-ORBoc-Phe-MeCys-OR Boc-Phe-MeMet-OR Boc-Tyr Boc-Tyr-MeSer-ORBoc-Tyr-MeThr-OR Boc-Tyr-MeCys-OR Boc-Tyr-MeMet-OR Boc-SerBoc-Ser-MeSer-OR Boc-Ser-MeThr-OR Boc-Ser-MeCys-OR Boc-Ser-MeMet-ORBoc-Thr Boc-Thr-MeSer-OR Boc-Thr-MeThr-OR Boc-Thr-MeCys-ORBoc-Thr-MeMet-OR Boc-Cys Boc-Cys-MeSer-OR Boc-Cys-MeThr-ORBoc-Cys-MeCys-OR Boc-Cys-MeMet-OR Boc-Met Boc-Met-MeSer-ORBoc-Met-MeThr-OR Boc-Met-MeCys-OR Boc-Met-MeMet-OR Boc-AspBoc-Asp-MeSer-OR Boc-Asp-MeThr-OR Boc-Asp-MeCys-OR Boc-Asp-MeMet-ORBoc-Glu Boc-Glu-MeSer-OR Boc-Glu-MeThr-OR Boc-Glu-MeCys-ORBoc-Glu-MeMet-OR Boc-Asn Boc-Asn-MeSer-OR Boc-Asn-MeThr-ORBoc-Asn-MeCys-OR Boc-Asn-MeMet-OR Boc-Gln Boc-Gln-MeSer-ORBoc-Gln-MeThr-OR Boc-Gln-MeCys-OR Boc-Gln-MeMet-OR Boc-LysBoc-Lys-MeSer-OR Boc-Lys-MeThr-OR Boc-Lys-MeCys-OR Boc-Lys-MeMet-ORBoc-Trp Boc-Trp-MeSer-OR Boc-Trp-MeThr-OR Boc-Trp-MeCys-ORBoc-Trp-MeMet-OR Boc-His Boc-His-MeSer-OR Boc-His-MeThr-ORBoc-His-MeCys-OR Boc-His-MeMet-OR Boc-Arg Boc-Arg-MeSer-ORBoc-Arg-MeThr-OR Boc-Arg-MeCys-OR Boc-Arg-MeMet-OR Boc-ProBoc-Pro-MeSer-OR Boc-Pro-MeThr-OR Boc-Pro-MeCys-OR Boc-Pro-MeMet-ORBoc-Orn Boc-Orn-MeSer-OR Boc-Orn-MeThr-OR Boc-Orn-MeCys-ORBoc-Orn-MeMet-OR MeAsp-OR MeGlu-OR MeAsn-OR Boc-Gly Boc-Gly-MeAsp-ORBoc-Gly-MeGlu-OR Boc-Gly-MeAsn-OR Boc-Ala Boc-Ala-MeAsp-ORBoc-Ala-MeGlu-OR Boc-Ala-MeAsn-OR Boc-Val Boc-Val-MeAsp-ORBoc-Val-MeGlu-OR Boc-Val-MeAsn-OR Boc-Leu Boc-Leu-MeAsp-ORBoc-Leu-MeGlu-OR Boc-Leu-MeAsn-OR Boc-Ile Boc-Ile-MeAsp-ORBoc-Ile-MeGlu-OR Boc-Ile-MeAsn-OR Boc-Phe Boc-Phe-MeAsp-ORBoc-Phe-MeGlu-OR Boc-Phe-MeAsn-OR Boc-Tyr Boc-Tyr-MeAsp-ORBoc-Tyr-MeGlu-OR Boc-Tyr-MeAsn-OR Boc-Ser Boc-Ser-MeAsp-ORBoc-Ser-MeGlu-OR Boc-Ser-MeAsn-OR Boc-Thr Boc-Thr-MeAsp-ORBoc-Thr-MeGlu-OR Boc-Thr-MeAsn-OR Boc-Cys Boc-Cys-MeAsp-ORBoc-Cys-MeGlu-OR Boc-Cys-MeAsn-OR Boc-Met Boc-Met-MeAsp-ORBoc-Met-MeGlu-OR Boc-Met-MeAsn-OR Boc-Asp Boc-Asp-MeAsp-ORBoc-Asp-MeGlu-OR Boc-Asp-MeAsn-OR Boc-Glu Boc-Glu-MeAsp-ORBoc-Glu-MeGlu-OR Boc-Glu-MeAsn-OR Boc-Asn Boc-Asn-MeAsp-ORBoc-Asn-MeGlu-OR Boc-Asn-MeAsn-OR Boc-Gln Boc-Gln-MeAsp-ORBoc-Gln-MeGlu-OR Boc-Gln-MeAsn-OR Boc-Lys Boc-Lys-MeAsp-ORBoc-Lys-MeGlu-OR Boc-Lys-MeAsn-OR Boc-Trp Boc-Trp-MeAsp-ORBoc-Trp-MeGlu-OR Boc-Trp-MeAsn-OR Boc-His Boc-His-MeAsp-ORBoc-His-MeGlu-OR Boc-His-MeAsn-OR Boc-Arg Boc-Arg-MeAsp-ORBoc-Arg-MeGlu-OR Boc-Arg-MeAsn-OR Boc-Pro Boc-Pro-MeAsp-ORBoc-Pro-MeGlu-OR Boc-Pro-MeAsn-OR Boc-Orn Boc-Orn-MeAsp-ORBoc-Orn-MeGlu-OR Boc-Orn-MeAsn-OR MeGln-OR MeLys-OR MeTrp-OR Boc-GlyBoc-Gly-MeGln-OR Boc-Gly-MeLys-OR Boc-Gly-MeTrp-OR Boc-AlaBoc-Ala-MeGln-OR Boc-Ala-MeLys-OR Boc-Ala-MeTrp-OR Boc-ValBoc-Val-MeGln-OR Boc-Val-MeLys-OR Boc-Val-MeTrp-OR Boc-LeuBoc-Leu-MeGln-OR Boc-Leu-MeLys-OR Boc-Leu-MeTrp-OR Boc-IleBoc-Ile-MeGln-OR Boc-Ile-MeLys-OR Boc-Ile-MeTrp-OR Boc-PheBoc-Phe-MeGln-OR Boc-Phe-MeLys-OR Boc-Phe-MeTrp-OR Boc-TyrBoc-Tyr-MeGln-OR Boc-Tyr-MeLys-OR Boc-Tyr-MeTrp-OR Boc-SerBoc-Ser-MeGln-OR Boc-Ser-MeLys-OR Boc-Ser-MeTrp-OR Boc-ThrBoc-Thr-MeGln-OR Boc-Thr-MeLys-OR Boc-Thr-MeTrp-OR Boc-CysBoc-Cys-MeGln-OR Boc-Cys-MeLys-OR Boc-Cys-MeTrp-OR Boc-MetBoc-Met-MeGln-OR Boc-Met-MeLys-OR Boc-Met-MeTrp-OR Boc-AspBoc-Asp-MeGln-OR Boc-Asp-MeLys-OR Boc-Asp-MeTrp-OR Boc-GluBoc-Glu-MeGln-OR Boc-Glu-MeLys-OR Boc-Glu-MeTrp-OR Boc-AsnBoc-Asn-MeGln-OR Boc-Asn-MeLys-OR Boc-Asn-MeTrp-OR Boc-GlnBoc-Gln-MeGln-OR Boc-Gln-MeLys-OR Boc-Gln-MeTrp-OR Boc-LysBoc-Lys-MeGln-OR Boc-Lys-MeLys-OR Boc-Lys-MeTrp-OR Boc-TrpBoc-Trp-MeGln-OR Boc-Trp-MeLys-OR Boc-Trp-MeTrp-OR Boc-HisBoc-His-MeGln-OR Boc-His-MeLys-OR Boc-His-MeTrp-OR Boc-ArgBoc-Arg-MeGln-OR Boc-Arg-MeLys-OR Boc-Arg-MeTrp-OR Boc-ProBoc-Pro-MeGln-OR Boc-Pro-MeLys-OR Boc-Pro-MeTrp-OR Boc-OrnBoc-Orn-MeGln-OR Boc-Orn-MeLys-OR Boc-Orn-MeTrp-OR MeHis-OR MeArg-ORMeOrn-OR Boc-Gly Boc-Gly-MeHis-OR Boc-Gly-MeArg-OR Boc-Gly-MeOrn-ORBoc-Ala Boc-Ala-MeHis-OR Boc-Ala-MeArg-OR Boc-Ala-MeOrn-OR Boc-ValBoc-Val-MeHis-OR Boc-Val-MeArg-OR Boc-Val-MeOrn-OR Boc-LeuBoc-Leu-MeHis-OR Boc-Leu-MeArg-OR Boc-Leu-MeOrn-OR Boc-IleBoc-Ile-MeHis-OR Boc-Ile-MeArg-OR Boc-Ile-MeOrn-OR Boc-PheBoc-Phe-MeHis-OR Boc-Phe-MeArg-OR Boc-Phe-MeOrn-OR Boc-TyrBoc-Tyr-MeHis-OR Boc-Tyr-MeArg-OR Boc-Tyr-MeOrn-OR Boc-SerBoc-Ser-MeHis-OR Boc-Ser-MeArg-OR Boc-Ser-MeOrn-OR Boc-ThrBoc-Thr-MeHis-OR Boc-Thr-MeArg-OR Boc-Thr-MeOrn-OR Boc-CysBoc-Cys-MeHis-OR Boc-Cys-MeArg-OR Boc-Cys-MeOrn-OR Boc-MetBoc-Met-MeHis-OR Boc-Met-MeArg-OR Boc-Met-MeOrn-OR Boc-AspBoc-Asp-MeHis-OR Boc-Asp-MeArg-OR Boc-Asp-MeOrn-OR Boc-GluBoc-Glu-MeHis-OR Boc-Glu-MeArg-OR Boc-Glu-MeOrn-OR Boc-AsnBoc-Asn-MeHis-OR Boc-Asn-MeArg-OR Boc-Asn-MeOrn-OR Boc-GlnBoc-Gln-MeHis-OR Boc-Gln-MeArg-OR Boc-Gln-MeOrn-OR Boc-LysBoc-Lys-MeHis-OR Boc-Lys-MeArg-OR Boc-Lys-MeOrn-OR Boc-TrpBoc-Trp-MeHis-OR Boc-Trp-MeArg-OR Boc-Trp-MeOrn-OR Boc-HisBoc-His-MeHis-OR Boc-His-MeArg-OR Boc-His-MeOrn-OR Boc-ArgBoc-Arg-MeHis-OR Boc-Arg-MeArg-OR Boc-Arg-MeOrn-OR Boc-ProBoc-Pro-MeHis-OR Boc-Pro-MeArg-OR Boc-Pro-MeOrn-OR Boc-OrnBoc-Orn-MeHis-OR Boc-Orn-MeArg-OR Boc-Orn-MeOrn-OR

The term “amino acid side chain” is used herein in its broadest senseand refers to the side chains of both L- and D-amino acids including the20 common amino acids such as alanine, arginine, asparagine, asparticacid, cysteine, glutamic acid, glutamine, glycine, histidine,isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine,threonine, tryptophan, tyrosine and valine; the less common amino acidsbut known derivatives such as cystine, 5-hydroxylysine,4-hydroxyproline, α-aminoadipic acid, α-amino-n-butyric acid,3,4-dihydroxyphenylalanine, homoserine, α-methylserine, ornithine,pipecolic acid and thyroxine; and any amino acid having a molecularweight less than about 500.

EXAMPLES

The invention will now be described with reference to the followingexamples. These examples are not to be construed as limiting theinvention in any way.

All melting points are uncorrected and were recorded on a microscopehot-stage apparatus. Infrared spectra were recorded on a FTIRspectrometer, using a diffuse reflectance accessory with KBr background.Standard pulse sequences (HMBC, HSQC, COSY, and DEPT) were used toidentify compound 66. Electrospray mass spectra (E.S.M.S.) were obtainedon a triple quadrupole mass spectrometer using water/methanol/aceticacid (0:99:1 or 50:50:1) mixtures as the mobile phase. Low resolutionmass spectra (e.i.) were performed. Other low and high resolution massspectra (l.s.i.m.s.) were measured. Ethyl acetate and hexane used forchromatography were distilled prior to use. All solvents were purifiedby distillation. For dry solvents, procedures from Perrin and Armarego²⁹were followed. Dry dichloromethane was distilled and stored over Lindetype 4 Å molecular sieves. All other reagents and solvents were purifiedor dried as described by Perrin and Armarego.²⁹

Example 1 Serine, Threonine and Tyrosine

The formation of the 5-oxazolidinones of serine and threonine iscomplicated by participation of the sidechain hydroxyls to formoxazolidines as shown in structures (5) and (6) and, in the case ofthreonine, this intermediate was also produced in the attemptedreductive cleavage²⁷. Thus, for sidechain protection, several strategieswere considered.

While a number of protected derivatives, of serine and threonine areknown ³⁰⁻³³ and were considered, in the end, the simple expedient ofacetylation fulfilled all the objectives. L-Threonine (8) was used toprepare O-acetyl threonine (9) in high yield according to the method ofWilchek and Patchornik (Scheme 10).³⁴ This procedure was equallysuccessful with L-serine (10) in providing the acetate (11).

The formation of the 5-oxazolidinones (14) (87%) and (15) (91%) usingthe intermediates (12) and (13), respectively proceeded in high yield.Reductive cleavage gave the N-methyl-O-acetyl amino acids (16) (74%) and(17) (80%) as their dicyclohexylamine salts. These acetates are insuitable form for use in solution and solid phase coupling proceduresbut the deprotection procedure required additional examination.Hydrolysis of the acetate esters under basic conditions has beenreported in relation to serine derivatives³⁵ but was unsuccessful inthis study with the threonine acetate (17a). Attempted base hydrolysesof the threonine acetate (17a) always resulted in isolation of thestarting material and this was attributed to the in situ formation ofthe tetrahedral intermediate (18) (FIG. 2) which survived the hydrolyticconditions and returned the starting material upon acidic work-up.

Conversely, aqueous acidic conditions and mild heating (Scheme 10)removed the acetate in high yield to give the alcohol (19) (88%). Thesame sequence of reactions works well for the serine intermediates (14),(16) and (20). Confirmation that this synthetic sequence did not reducethe optical purity was obtained by hydrogenolysis of the threoninecarbamate (19) (Scheme 11). The isolated N-methyl-L-Threonine (21) hadan optical rotation of [α]_(D) −14° (c=1, 6M HCl) which matchedpreviously reported values.²⁸ Thus, N-methyl serine and threonine withand without sidechain protection, are available by a side chain O-acetylprotection strategy.

Tyrosine forms the expected oxazolidinone without sidechain protectionbut the yields for its formation (37%) and subsequent reductive cleavage(60%) were lower than desired. Given the success of sidechainacetylation in the serine and threonine manipulations, a similarstrategy was attempted with tyrosine. Solubility problems wereencountered with the Fmoc carbamate of tyrosine and its conversion tothe corresponding acetate. The commercially available tyrosine benzylether (22) suited the oxazolidinone chemistry, and the oxazolidinone(23) was isolated in 86% yield (Scheme 12). Reductive cleavage then gavethe N-methyl tyrosine (24) in 70% yield: a substantial improvementcompared with the previous sequence in which the hydroxy group wasunprotected.

The tyrosine benzyl carbamate (25)³⁶ was also converted to theoxazolidinone (26) (89%) and reductive cleavage afforded the N-methyltyrosine O-acetate (27) (88%). Formation of the N-methyl tyrosine (27)represents a 40% improvement compared to the tyrosine sequence in whichthe hydroxy group was unprotected.

(S)-3-Benzyloxycarbonyl-4-acetoxymethyloxazolidin-5-one (14)

To a sample of the carbamate (12) (1.11 g, 4.0 mmol) in toluene (50 ml)was added camphorsulfonic acid (70 mg) and dry paraformaldehyde (1.0 g).The reaction mixture was then heated to reflux for 30-60 mins [monitoredby TLC (40% ethyl acetate-hexane)]. The mixture was cooled, filtered toremove solids and diluted with ether (150 ml). The ethereal solution waswashed with 2.5% aqueous sodium bicarbonate solution (4×30 ml).

The combined aqueous layers were extracted with ether (30 ml) and thecombined ethereal layers were dried (MgSO₄), filtered and concentratedin vacuo to give the oxazolidinone (14) as an oil (1.01 g, 87%). A smallsample was further purified by flash chromatography on silica elutingwith 30% ethyl acetate-hexane. [α]_(D) ²⁴ +110.7° (c 1.0, CHCl₃). ¹H NMR(300 MHz, CDCl₃) 7.33 (s, 5H), 5.51 (bs, 1H), 5.20 (d, 1H, J=3.8 Hz),5.16 (s, 2H), 4.61-4.58 (m, 1H) 4.42-4.32 (m, 2H), 1.99 (s, 3H). ¹³C NMR(75 MHz, CDCl₃) 169.85, 152.30, 135.07, 128.57, 128.25, 78.39, 68.04,62.19, 54.41, 20.44. IR (NaCl) ν 3090, 3065, 3034 and 3010 (CH,aromatic), 3000-2900 (CH, saturated), 1807 (C═O, oxazolidinone), 1746(C═O, acetate), 1719 (C═O, carbamate), 1500, 1452, 1419, 1359, 1315,1290, 1234, 1170, 1130, 1060, 1034, 969, 945, 765, 699 cm⁻¹. Anal. Calcdfor C₁₄H₁₅NO₆: C, 57.34; H, 5.16; N, 4.78. Found: C, 57.54; H, 5.26; N,4.96.

(4S)-3-Benzyloxycarbonyl-4-[(1S)-acetoxyethyl]oxazolidin-5-one (15)

To a sample of the carbamate (13) (1.47 g, 5 mmol) in toluene (50 ml)was added camphorsulfonic acid (70 mg) and dry paraformaldehyde (1.0 g).The reaction mixture was then heated to reflux for 30-60 mins [monitoredby TLC (40% ethyl acetate-hexane)). The mixture was cooled, filtered toremove solids and diluted with ether (150 ml). The ethereal solution waswashed with 2.5% aqueous sodium bicarbonate solution (4×30 ml). Thecombined aqueous layers were extracted with ether (30 ml) and thecombined ethereal layers were dried (MgSO₄), filtered and concentratedin vacuo to give the oxazolidinone (15) as an oil (1.38 g, 91%). A smallsample was further purified by flash chromatography on silica elutingwith 30% ethyl acetate-hexane. [α]_(D) ²⁴ +120.7° (c 2.0, CHCl₃). ¹H NMR(300 MHz, CDCl₃) 7.32 (s, 5H), 5.70 (bs, 1H), 5.29-5.18 (m, 4H), 4.41(bs, 1H), 1.97 (s, 3H), 1.34-1.31 (m, 3H). ¹³C NMR (75 MHz, CDCl₃)δ_(C)170.14, 169.19, 153.83, 134.98, 128.59, 128.37, 78.98, 70.59,68.34, 59.11, 20.76, 16.63. IR (NaCl) ν 3092, 3066 and 3033 (CH,aromatic), 3000-2900 (CH, saturated), 1808 (C═O, oxazolidinone), 1742(C═O, acetate), 1719 (C═O, carbamate), 1498, 1454, 1409, 1360, 1328,1232, 1169, 1124, 1041, 955, 897, 753, 700 cm⁻¹ MS (l.s.i.m.s.) m/z 308(M+1, 90%), 289, (50), 264 (100). HRMS calcd for C₁₅H₁₇NO₆ (M+1)308.1134 found 308.1142. Anal. Calcd for C₁₅H₁₇NO₆: C, 58.63; H, 5.58;N, 4.56. Found: C, 58.62; H, 5.63; N, 4.71.

N-Benzyloxycarbonyl-N-methyl-L-serine-O-acetate (16)

A sample of the oxazolidinone (14) (1.18 g, 4.0 mmol) was dissolved inchloroform (20 ml) at room temperature and triethylsilane (1.89 ml) wasadded followed by trifluoroacetic acid (20 ml) and the reaction mixturewas left to stand for 3-4 d. The reaction mixture was concentrated underreduced pressure. To the residue was added toluene (50 ml) and themixture was again concentrated in vacuo. This procedure was repeatedwith more toluene (50 ml). The residue was then diluted with ether andextracted with saturated aqueous sodium bicarbonate solution (4×30 ml).The combined aqueous extracts were washed with ether and then acidifiedto pH 2 with 5 M hydrochloric acid. The aqueous phase was then extractedwith ether (3×50 ml). The combined ethereal extracts were dried (MgSO₄),filtered and evaporated under reduced pressure to approximately 20 mlvolume. Dicyclohexylamine (DCHA) (0.8 ml) was added and any solid, whichformed immediately, was filtered off. The clear filtrate was left tostand overnight during which the N-methyl serine acetate (16)precipitated as its DCHA salt. (1.40 g, 74%). Mp 135-147° C. [α]_(D) ²⁵−8° (c 2.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) 9.15 (bs, 2H), 7.33-7.24 (m,5H), 5.21-4.99 (m, 2H), 4.84 (td, 1H, J═10.0, 4.2 Hz), 4.55 (dt, 1H,J═11.8, 3.8 Hz), 4.42-4.26 (m, 1H), 2.97-2.88 (m, 5H), 1.97-1.13 (m,23H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ6172.30, 172.14, 170.85,156.97, 156.77, 137.01, 128.37, 127.81, 127.58, 127.52, 66.97, 66.89,62.54, 62.48, 59.99, 59.75, 52.75, 31.14, 28.95, 25.04, 24.67, 20.83,20.74. IR (KBr disk)ν 3062, 3034 and 3005 (CH, aromatic), 3000-2800 (CH,saturated), 2476 and 2417 (NH₂ ⁺), 1738 (C═O, acetate), 1693 (C═O,carbamate), 1641 (CO₂ ⁻), 1566, 1441, 1392, 1370, 1345, 1312, 1286,1250, 1149, 1075, 696 cm⁻¹. Anal. Calcd for C₂₆H₄₀N₂O₆: C, 65.52; H,8.46; N, 5.88. Found: C, 65.52; H, 8.65; N, 5.86.

N-Benzyloxycarbonyl-N-methyl-L-threonine-O-acetate (17)

A sample of the oxazolidinone (15) (1.22 g, 4.0 mmol) was dissolved inchloroform (20 ml) at room temperature and triethylsilane (1.89 ml) wasadded followed by trifluoroacetic acid (20 ml) and the reaction mixturewas left to stand for 3-4 d. The reaction mixture was concentrated underreduced pressure. To the residue was added toluene (50 ml) and themixture was again concentrated in vacuo. This procedure was repeatedwith more toluene (50 ml). The residue was then diluted with ether andextracted with saturated aqueous sodium bicarbonate solution (4×30 ml).The combined aqueous extracts were washed with ether and then acidifiedto pH 2 with 5 M hydrochloric acid. The aqueous phase was then extractedwith ether (3×50 ml). The combined ethereal extracts were dried (MgSO₄),filtered and evaporated under reduced pressure to approximately 20 mlvolume. Dicyclohexylamine (DCHA) (0.8 ml) was added and any solid, whichformed immediately, was filtered off. The filtrate solution was left tostand overnight during which the N-methyl threonine acetate (17)precipitated as its DCHA salt (1.57 g, 80%).

N-Benzyloxycarbonyl-N-methyl-L-serine (20)

A sample of the serine DCHA salt (16) (970 mg, 2.0 mmol) was suspendedin a mixture of dioxane and 2M hydrochloric acid (20 ml, 1:1) withstirring. The mixture was then heated to 60° C. for ca. 30 h (TLC). Thereaction mixture was then diluted with water (300 ml) and extracted withether (3×100 ml). The combined organic phases were dried (MgSO₄),filtered and evaporated at reduced pressure to give the N-methyl serine(19) as an oil (480 mg, 95%), which was identical in all respects withpreviously reported material.²⁸

N-Benzyloxycarbonyl-N-methyl-L-threonine (19)

A sample of the threonine DCHA salt (17) (1.04 g, 2.0 mmol) wassuspended in a mixture of dioxane and 2M hydrochloric acid (20 ml, 1:1)with stirring. The mixture was then heated to 60° C. for ca. 30 h (TLC).The reaction mixture was then diluted with water (300 ml) and extractedwith ether (3×100 ml). The combined organic phases were dried (MgSO₄),filtered and evaporated at reduced pressure to give the N-methylthreonine (19) as an oil (520 mg, 97%), which was identical in allrespects with previously reported material.²⁸

N-Methyl-L-threonine (21)

A small sample of the carbamate (19) was hydrogenolysed over 10%palladium on charcoal catalyst.²⁸ The material isolated had [α]_(D) ²⁵−14° (c, 0.5 in 6 M HCl) which was identical with authentic material.²⁸

(S)-3-(Carbonyl-9H-fluoren-9-ylmethoxy)-4-(4-benzyloxybenzyl)-oxazolidin-5-one(23)

To a sample of the carbamate (22) (470 mg, 0.9 mmol) in toluene (150 ml)was added camphorsulfonic acid (66 mg). The reaction mixture was thenheated to reflux for 4 h during which dry paraformaldehyde (500 mg) wasadded in small portions down the condenser. The mixture was then cooled,filtered to remove solids and the filtrate was evaporated under reducedpressure. The residue was taken up in ethyl acetate and washed withsaturated aqueous sodium bicarbonate solution (3×30 ml). The organiclayer was dried (MgSO₄), filtered and concentrated in vacuo. The residuewas purified by flash chromatography on silica eluting with 25% ethylacetate-hexane to give the oxazolidinone (23) as a foam (405 mg, 86%).[α]_(D) ²² +132.5° (c 1.0, Et₂O). ¹H NMR (300 MHz, CDCl₃) (rotamers)7.77-6.60 (m, 17H), 5.11 (brs, 1H), 4.99-4.95 (m, 1H), 4.73-4.64 (m,1H), 4.47 (m, 0.5H), 4.27-4.19 (m, 1H), 4.10 (m, 1H), 3.94 (m, 0.5H),3.32-2.35 (m, 2H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ171.54, 157.84,151.84, 143.09, 141.12, 136.50, 126.19, 130.31, 128.25, 127.67, 127.17,126.93, 124.20, 119.87, 119.80, 114.76, 77.49, 69.56, 67.10, 66.31,56.05, 46.93, 34.20. IR (KBr disk) ν 3034 (CH, aromatic), 3000-2800 (CH,saturated), 1800 (C═O, oxazolidinone), 1717 (C═O, carbamate), 1610,1511, 1451, 1422, 1357, 1300, 1242, 1177, 1159, 1129, 1052, 1024, 830,759, 741, 696 cm⁻¹. HRMS calcd for C₃₂H₂₇NO₅ (M⁺) 505.1968, found505.1891.

(S)-3-Benzyloxycarbonyl-4-(4-acetoxybenzyl)-oxazolidin-5-one (26)

To a sample of the carbamate (25) (2.90 g, 7.9 mmol) in toluene (50 ml)was added camphorsulfonic acid (200 mg). To the reaction mixture wasadded dry paraformaldehyde (3.0 g) and the mixture was heated to refluxfor 1 h. The mixture was then cooled, filtered to remove solids and thefiltrate was evaporated under reduced pressure. The residue was taken upin ether (100 ml). The ether layer was washed with 5% sodium carbonatesolution (3×50 ml) followed by water and then brine. The 30 organiclayer was dried (MgSO₄), filtered and concentrated in vacuo. The residue(2.80 g) was purified by flash chromatography on silica eluting with 20%ethyl acetate-hexane to give the oxazolidinone (26) as a clearcolourless oil (2.61 g, 89%). [α]_(D) ²⁴ +172.3° (c 1.0, CHCl₃). ¹H NMR(300 MHz, CDCl₃) 7.35-7.29 and 7.21-6.91 (2m, 9H), 5.28-5.14 (m, 3H),4.49 (brs, 1H), 4.32 (d, 1H, J=3.9 Hz), 3.42-3.08 (m, 2H), 2.23 (s, 3H).¹³C NMR (75 MHz, CDCl₃) δ 171.51, 168.95, 152.06, 149.93, 135.37,131.91, 130.42, 128.51, 128.25, 121.70, 77.78, 67.62, 56.11, 35.33,34.31, 20.85. IR (NaCl)ν 3100, 3062 and 3034 (CH, aromatic), 3000-2800(CH, saturated), 1800 (C═O, oxazolidinone), 1760 (C═O, acetate), 1716(C═O, carbamate), 1604, 1506, 1416, 1361, 1310, 1202, 1126, 1049, 1013,912, 843, 754 cm⁻¹. Anal. Calcd for C₂₀H₁₉NO₆: C, 65.03; H, 5.18; N,3.79. Found: C, 64.89; H, 5.35; N, 3.87.

N-(Carbonyl-9H-fluoren-9-ylmethoxy)-N-methyl-L-tyrosine-O-benzyl ether(24)

A sample of the oxazolidinone (23) (95 mg, 0.2 mmol) was dissolved indichloromethane (5 ml) at room temperature and triethylsilane (270 μl,1.7 mmol) was added followed by trifluoroacetic acid (1.2 ml, 10.5 mmol)and the reaction mixture was left to stand for 2 d. The reaction mixturewas concentrated under reduced pressure. To the residue was addeddichloromethane (5 ml) and the mixture was again concentrated in vacuo.This procedure was repeated with toluene (5 ml) until traces oftrifluoroacetic acid were removed. The residue was then diluted withethyl acetate and extracted with saturated aqueous sodium bicarbonatesolution (3×30 ml). The combined aqueous extracts were washed with etherand then acidified to pH 2 with 2 M hydrochloric acid. The aqueous phasewas then extracted with ethyl acetate (3×50 ml). The combined organicextracts were dried (MgSO₄), filtered and evaporated under reducedpressure. The residue was purified by flash chromatography eluting with95:4:1 dichloromethane/methanol/acetic acid to yield the N-methyl acid(24) as a white foam (67 mg, 70%). [α]_(D) ²⁴ −1.6° (c 0.5, Et₂O). R_(f)0.3 (95:4:1 dichloromethane/methanol/acetic acid). ¹H NMR (300 MHz,CDCl₃) (rotamers) 7.74-6.46 (m, 17H), 4.90-4.07 (m, 6H), 3.27 and3.08-2.96 and 2.66-2.63 (dd and 2m, 1H and 2H, J=4.8, 14.4 Hz), 2.78 and2.74 (2s, 3H). ¹C NMR (75 MHz, CDCl₃) (rotamers) δ 174.21, 156.70,155.85, 154.28, 143.41, 143.31, 140.93, 129.57, 128.24, 127.55, 126.73,124.64, 124.31, 119.62, 115.20, 67.62, 67.10, 60.59, 60.26, 46.81,46.69, 33.61, 33.46, 31.92.

N-Benzyloxycarbonyl-N-methyl-L-tyrosine-O-acetate (27)

A sample of the oxazolidinone (26) (1.50 g, 4.1 mmol) was dissolved inchloroform (20 ml) at room temperature and triethylsilane (1.9 ml) wasadded followed by trifluoroacetic acid (20 ml) and the reaction mixturewas left to stand for 4 d. The reaction mixture was diluted with tolueneand concentrated under reduced pressure. This procedure was repeatedwith a further aliquot of toluene (50 ml). The residue was then dilutedwith ether and extracted with 5% sodium carbonate solution (4×20 ml).The combined aqueous extracts were washed with ether and then acidifiedto pH 2 with 5 M hydrochloric acid. The aqueous phase was then extractedwith dichloromethane (3×50 ml). The combined extracts were dried(MgSO₄), filtered and evaporated under reduced pressure to afford aclear oil (1.33 g, 88%). A sample of the oil was converted to thet-butylamine salt by dissolution in ether and addition of t-butylamine(1.1 equiv.) followed by hexane until the solution turned slightlycloudy. The turbid solution was then left to stand at room temperaturefor 4 h and then at 0° C. overnight during which the N-methyl tyrosineacetate (27) precipitated as its t-butylammonium salt. Mp 54-60° C.[α]_(D) ²⁴ −34.3° (c 1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) (rotamers)7.74 (brs, 3H), 7.29-6.88 (m, 9H), 5.01-4.64 (m, 3H), 3.38-3.28 and3.00-2.77 (2m, 5H), 2.26 (s, 3H), 1.23 (s, 9H). ¹³C NMR (75 MHz, CDCl₃)(rotamers) δ175.95, 169.51, 156.89, 156.50, 148.99, 136.78, 136.66,136.60, 136.50, 129.68, 128.39, 127.76, 127.45, 121.32, 66.97, 66.86,62.64, 62.34, 51.23, 35.51, 35.05, 31.90, 31.08, 27.51, 21.10. IR (KBrdisk). ν3121, 3064 and 3033 (CH, aromatic), 3000-2800 (CH, saturated),2622 and 2529, ⁺NH₃, 1760 (C═O, acetate), 1674 (C═O, carbamate), 1592(CO₂ ⁻), 1507, 1448, 1377, 1314, 1209, 1194, 1134, 750, 695, 639 cm⁻¹.Anal. Calcd for C₂₄H₃₂N₂O₆: C, 64.85; H, 7.26; N, 6.30. Found: C, 64.71;H, 7.39; N, 6.41.

Example 2 Cysteine and Cystine

The synthesis of the sulfur bearing N-methyl amino acids gave mixedresults using the 5-oxazolidinone route (FIG. 3).²⁸ The cysteinecarbamate (28a) gave the oxazolidinone (29a) in only 3% yield.Methionine, on the other hand, gave the oxazolidinone (29b) in 91%. Thereductive cleavage of the oxazolidinone (29a) gave the thiazolidine (30)exclusively indicating the requisite iminium ion had been formed and wasthen intercepted intramolecularly by the thiol. The methionineintermediate (29b) gave a mixture of products.

To lessen the nucleophilicity of the thiol, the S-acetyl cysteinederivative (31a)^(37,38) was prepared and this underwent oxazolidinationin moderate yield (51%) (Scheme 13). However, attempted reductivecleavage of the oxazolidinone (32) gave no N-methyl products uponworkup. Thus the S-benzyl cysteine (31c)³⁹ was converted to theoxazolidinone (33) in high yield (89%) and subsequent reductive cleavagewith trifluoroacetic acid and triethylsilane gave the expected N-methylamino acid (34) (70%). Removal of the S-benzyl group in any subsequentsequence may present problems given the preferred method fordebenzylation involves treatment with HF.³⁸ Thus protection using a(S-PMB (p-methoxybenzyl) acid (31d) was proposed, as the ultimateremoval of the PMB ether can be effected with refluxing trifluoroaceticacid.⁴⁰ The ether was as prepared but attempts to convert it to thecorresponding oxazolidinone resulted in decomposition.

However, the formation of N-methyl cysteine is performed efficiently bythe related method of Yamashiro et al³⁹ which involves the reaction ofcysteine with paraformaldehyde to give a thiazolidine carboxylic acid. Adissolving metal reductive cleavage of the thiazolidine ring generatesN-methyl cysteine, which can then be converted in many ways to a rangeof synthetically useful intermediates including the S-benzyl carbamate(34).

During the studies to solve the cysteine manipulation problems, the useof the cystine carbamate (35) (Scheme 14) was also trialed.Oxazolidinone formation gave the dimeric structure (36) as a solid(33%). However, the reductive cleavage resulted in isolation of thethiazolidine (30). Evidently, the disulfide bridge is cleaved initiallygiving the cysteine oxazolidinone (31a) in situ. This was thentransformed into the expected iminium ion, which reacts with the thiol,as before, to give the thiazolidine (30).

It was previously reported²⁷ that oxazolidination of cysteine led to theformation of the dimeric structure (37). In reality this dimericstructure is a proton sharing aggregate of two thiazolidines (30) thatforms in the ESMS. The proposal of the structure (37) was based on theobservance in the electrospray mass spectrum of m/z 535. However,further analysis of the cysteine product revealed the appearance of them/z 535 peak was concentration dependent. Furthermore, while the ESMS.of the putative aggregate also exhibited peaks at m/z 557 and 268corresponding to M+Na and M+2/2, that same spectrum did not show a peakat m/z 279 for M+Na+H/2. The m/z 268 peak is revealed as the M+H ion forthe thiazolidine (30). Thus, the dimer (37) is not formed in thecysteine oxazolidination; only the thiazolidine (30) is formed in thatreaction.

(R)-3-Benzyloxycarbonyl-4-(acetylthiomethyl)oxazolidin-5-one (32)

In a round-bottomed flask fitted with a Dean-Stark apparatus, a mixtureof the S-acetyl cysteine (31b) (1.0 g, 3.4 mmol), paraformaldehyde (450mg) and camphorsulfonic acid (40 mg) was suspended in benzene (30 ml).The mixture was heated to reflux for 3 h (monitored by TLC). Thereaction mixture was then concentrated at reduced pressure. The residuewas taken up in ethyl acetate and the organic layer was washed withsaturated aqueous sodium bicarbonate solution to remove acidic material.The organic layer was dried (MgSO₄), filtered and evaporated in vacuo.The residue was purified by column chromatography, eluting with 50%ethyl acetate-hexane to afford the oxazolidinone (32) as an oil (540 mg,51%). [α]_(D) ²⁵ +101.0° (c 0.9, CHCl₃). ¹H NMR (300 MHz, CDCl₃)7.35-7.30 (m, 5H), 5.44 (bs, 1H), 5.22-5.14 (m, 3H), 4.52 (bs, 1H), 3.65(dd, 1H, J=4.7, 14.2 Hz), 3.41-3.30 (m, 1H), 2.29 (s, 3H). ¹³C NMR (75MHz, CDCl₃) δ 193.03, 170.28, 152.39, 135.17, 128.51, 128.45, 128.24,127.82, 78.39, 68.03, 54.60, 30.36, 29.36. IR (NaCl) ν 3110, 3090, 3065and 3034 (CH, aromatic), 3000-2800 (CH, saturated), 1804 (C═O,oxazolidinone), 1714 (C═O, carbamate, acetate), 1500, 1412, 1357, 1290,1215, 1168, 1129, 1051, 1020, 966, 884, 764, 699, 620 cm⁻¹. Anal. Calcdfor C₁₄H₁₅NO₅S: C, 54.36; H, 4.89; N, 4.53; S, 10.37. Found: C, 54.47;H, 4.94; N, 4.32; S, 10.29.

(R)-3-Benzyloxycarbonyl-4-(phenylmethylthiomethyl)oxazolidin-5-one (33)

In a round-bottomed flask fitted with a Dean-Stark apparatus, a mixtureof the S-benzyl cysteine (31b) (1.0 g, 2.9 mmol), paraformaldehyde (450mg) and camphorsulfonic acid (50 mg) was suspended in benzene (30 ml).The mixture was heated to reflux (monitored by TLC for disappearance ofstarting material). The reaction mixture was then concentrated atreduced pressure. The residue was taken up in ethyl acetate and theorganic layer was washed with saturated aqueous sodium bicarbonatesolution to remove acidic material. The organic layer was dried (MgSO₄),filtered and evaporated in vacuo. The pale yellow syrupy residue waspurified by column chromatography, eluting with 20% ether-hexane then20-50% ethyl acetate-hexane to afford the oxazolidinone (33) as a clearcolourless oil (920 mg, 89%). [α]_(D) ²⁴ +102.3° (c 0.6, CHCl₃). ¹H NMR(300 MHz, CDCl₃) 7.34-7.20 (m, 10H), 5.50 (bs, 1H), 5.35 (d, 1H, J=4.1Hz), 5.16 (s, 2H), 4.50 (bs, 1H), 3.69 (d, 1H, J_(AB)=13.3 Hz), 3.65 (d,1H, J_(AB)=13.3 Hz), 3.37-2.89 (m, 2H). ¹³C NMR (75 MHz, CDCl₃) □171.28, 152.38, 137.45, 135.23, 128.95, 128.69, 128.55, 128.32, 127.28,78.77, 68.01, 56.05, 37.24, 31.90, 31.40. IR (NaCl)ν 3086, 3062, 3030and 3006 (CH, aromatic), 3000-2800 (CH, saturated), 1801 (C═O,oxazolidinone), 1717 (C═O, carbamate), 1495, 1452, 1413, 1357, 1290,1257, 1212, 1165, 1127, 1052, 1019, 961, 764, 699 cm⁻¹. Anal. Calcd forC₁₉H₁₉NO₄S: C, 63.85; H, 5.36; N, 3.92. Found: C, 63.59; H, 5.62; N,4.07.

N-Benzyloxycarbonyl-N-methyl-S-phenylmethyl-L-cysteine (34)⁵⁶

The oxazolidinone (33) (850 mg, 2.4 mmol) was taken up in chloroform (20ml). Triethylsilane (1.5 ml) was added followed by trifluoroacetic acid(20 ml) and the resulting mixture was left to stand for 2 d. Thereaction mixture was concentrated under reduced pressure. The residuewas diluted with excess saturated aqueous sodium bicarbonate solution.The aqueous phase was washed with ether and then acidified to pH 2 with2 M hydrochloric acid. The acidic layer was then extracted with ether.The ethereal extracts were dried (MgSO₄) and then treated withdicyclohexylamine (2.4 mmol) and the solution was stored overnight at 0°C. The crystalline precipitate that formed was filtered off at the pumpand dried to give the N-methyl-S-benzyl cysteine (34) as its DCHA salt(900 mg, 70%). Mp 105-107° C. [α]_(D) ²⁶ −56.0° (c 1.0, CHCl₃). ¹H NMR(300 MHz, CDCl₃) (rotamers) 7.35-7.17 (m, 10H), 5.25-5.03 (m, 2H), 4.76(dd, 1H, J=4.9, 10.6 Hz), 4.61 (dd, 1H, J=4.9, 10.5 Hz), 3.73-3.61 (m,2H), 3.12-3.05 (m, 1H), 2.89-2.83 (m, 5H), 2.71-2.65 (m, 1H), 1.91-1.03(m, 20H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ 173.99, 173.66, 157.07156.78, 138.49, 137.04, 136.85, 128.92, 128.73, 128.27, 127.70, 127.46,126.69, 67.04, 66.83, 60.31, 59.79, 52.37, 36.17, 35.76, 32.21, 31.75,30.62 30.29, 28.93, 28.81, 25.11, 24.67. IR (KBr disk)ν 3059 and 3029(CH, aromatic), 3000-2800 (CH, saturated), 2525 and 2466 (H₂N⁺), 1692(C═O, carbamate), 1624 (CO₂ ⁻), 1563, 1496, 1476, 1453, 1382, 1311,1293, 1169, 1128, 1024, 760, 700 cm⁻¹. Anal. Calcd for C₃₁H₄₄N₂O₄S: C,68.85; H, 8.20; N, 5.18; S, 5.93. Found: C, 68.91; H, 8.39; N, 5.05; S,5.85.

(4R,4′R)-3,3′-Bis-benzyloxycarbonyl-4,4′-[dithiobis(methylene)]bis-oxazolidin-5-one(36)

A mixture of the cystine carbamate (35) (3.0 g, 5.9 mmol),camphorsulfonic acid (40 mg), paraformaldehyde (2.0 g) and toluene (100ml) was heated to reflux (ca. 1.5 h, TLC). The reaction mixture was thenconcentrated under reduced pressure and the residue was filtered througha short column or plug of silica gel eluting with dichloromethane. Thefiltrate was concentrated in vacuo and the residual syrup wasrefrigerated at −5° C. overnight to initiate crystallisation. Themixture of syrup and most of the solid was taken up in hot ethersolution (small amounts of ethyl acetate can be added to facilitatedissolution). The solution was concentrated by boiling to ca. 15 ml andthen hexane (10 ml) was added. The solution was left to stand overnightat 0° C. The precipitate that formed was filtered off at the pump anddried to give the oxazolidinone (36) as a crystalline solid (1.05 g,33%). Mp 86-88° C. [α]_(D) ²³ +99.4° (c 1.0, CHCl₃). ¹H NMR (300 MHz,CDCl₃) 7.34-7.29 (m, 10H), 5.49-5.46 (m, 2H), 5.30 (bs, 2H), 5.15-5.12(m, 4H), 4.51 (brs, 2H), 3.47-3.12 (m, 4H). ¹³C NMR (75 MHz, CDCl₃) δ170.62, 152.15, 135.19, 128.66, 128.42, 78.43, 68.04, 55.07, 38.86,37.85. IR (KBr disk)ν 3090, 3065, and 3033 (CH, aromatic), 3000-2800(CH, saturated), 1796 (C═O, oxazolidinone), 1704 (C═O, carbamate), 1500,1453, 1431, 1362, 1296, 1268, 1213, 1175, 1159, 1125, 1055, 761, 700cm⁻¹. Anal. Calcd for C₂₄H₂₄N₂O₈S₂: C, 54.12; H, 4.54; N, 5.26; S,12.04. Found: C, 54.11; H, 4.46; N, 5.17; S, 11.96.

Attempted Reductive Cleavage of the Cystine Oxazolidinone (36)

The cystine oxazolidinone (36) (300 mg, 0.6 mmol) was taken up inchloroform (5 ml). Triethylsilane (750 μl) was added followed bytrifluoroacetic acid (5 ml) and the reaction mixture was left to standfor 2 d. Work-up of the reaction mixture as described for the N-methylcysteine (33) afforded the thiazolidine (30) as an oil (241 mg, 80%)identical in all respects to material previously reported.^(28, 56)

Example 3 Methionine

The methionine carbamate reacts well to form the oxazolidinone (29b)(FIG. 3), but the reductive cleavage was not successful and gave amixture of products. This was attributed to the sidechain thioetheracting as a cation scavenger (FIG. 4); a phenomenon, which is known inpeptide chemistry through the use of dimethyl sulfide.⁴¹

As with cysteine, the nucleophilicity of the thiomethyl group needed tobe ameliorated to prevent its participation in the reductive cleavage.The corresponding sulfoxide (38)⁴² (98%) was easily prepared (Scheme 15)by reaction of the oxazolidinone (29b) with m-chloroperoxybenzoic acid(mCPBA). Initial attempts to convert the methionine carbamate (28b) toits sulfoxide⁴³ were successful but the subsequent oxazolidination wascompromised by its poor solubility. The sulfoxide (38) was thenreductively cleaved in high yield (92%) to give the N-methyl amino acid(39). It was evident this procedure caused a small amount ofdeoxygenation of the sulfoxides (38) or (39) and so the procedureincluded treatment with hydrogen peroxide to reoxidize the thioether(40). The N-methyl methionine (40) was formed in 81% yield in a one-potprocedure, which included the ammonium iodide/dimethyl sulfidetreatment.

(S)-3-Carbonylbenzyloxy-4-(2-methanesulfinylethyl)-oxazolidin-5-one(38)⁴²

To a solution of the methionine oxazolidinone (29b) (3.0 g, 10.2 mmol)in dichloromethane (135 ml) was slowly added m-CPBA (1.74 g) and thereaction mixture was stirred at room temperature for 15 min. Thesolution was washed with sodium carbonate solution (3×40 ml, 10% w/v).The aqueous washings were extracted with dichloromethane (2×50 ml) andthe combined organic layers were dried (MgSO₄), filtered andconcentrated in vacuo to give the sulfoxide (35) as a clear colourlessgum. ¹H NMR (300 MHz, CDCl₃) 7.32 (s, 5H), 5.48-5.47 (m, 1H), 5.22-5.08(m, 3H), 4.38 (t, 1H, J=6.0 Hz), 2.75 (brs, 2H), 2.47 (s, 3H), 2.38-2.27(m, 2H). ¹³C NMR (75 MHz, CDCl₃) δ 171.09, 152.89, 152.83, 135.00,128.61, 128.33, 77.74, 68.13, 53.79, 53.68, 49.28, 38.54, 38.47, 24.39,24.08. IR (NaCl)ν 3038 (CH, aromatic), 3000-2900 (CH, saturated), 1796(C═O, oxazolidinone), 1714 (C═O, carbamate), 1502, 1413, 1356, 1317,1247, 1132, 1049, 753 cm⁻¹. HRMS calcd for C₁₄H₁₇NO₅S (M⁺) 311.0827,found 311.0832.

N-Benzyloxycarbonyl-N-methyl-L-methionine-d-sulfoxide (39a) andN-Benzyloxycarbonyl-N-methyl-L-methionine-l-sulfoxide (39b)

To a solution of the sulfoxides (38) (1.3 g, 4.2 mmol) in chloroform (22ml) was added triethylsilane (2.0 ml) and trifluoroacetic acid (22 ml).The reaction mixture was stirred at room temperature for 2 d and it wasthen concentrated at reduced pressure. The residue was taken up in ethylacetate and extracted with sodium carbonate solution (10% w/v, 4×15 ml).The combined aqueous extracts were washed with ethyl acetate and thenacidified with 5 M hydrochloric acid. The aqueous layer was thenextracted with dichloromethane (3×20 ml) and the combined organicextracts were dried (MgSO₄), filtered and concentrated in vacuo. Theresidue (1.22 g) was taken up in methanol (12 ml). To the methanolicsolution was added concentrated hydrochloric acid (20 μl). 30% Hydrogenperoxide was added dropwise until TLC indicated the presence of a singlecompound. The reaction mixture was concentrated at reduced pressure andthe residue was taken up in dichloromethane and washed with water. Thedichloromethane phase was then dried (MgSO₄), filtered and evaporated invacuo. The residue (1.22 g) was recrystallised from ethyl acetate-etherto give the sulfoxide (39a) as a solid (210 mg, 16%). Mp 145-148° C.[α]_(D) ²⁵ +21.0° (c 1.0, MeOH). ¹H NMR (300 MHz, CDCl₃) (rotamers)[(D₆)DMSO) 7.39-7.28 (m, 5H), 5.10-5.02 (m, 2H), 4.60-4.53 (m, 1H),2.82-2.70 (m, 3H), 2.67-2.58 (m, 2H), 2.51-2.47 (m, 3H), 2.22-2.20 (m,1H), 2.06-1.97 (m, 1H) ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ171.81,156.08, 155.59, 136.72, 128.43, 128.36, 127.84, 127.38, 66.48, 58.44,49.99, 38.11, 31.86, 31.30, 22.18, 21.50. IR (KBr). ν 3600-3200 (CO₂H),3063 and 3031 (CH, aromatic), 3000-2800 (CH, saturated), 1721 (CO,acid), 1691 (CO, carbamate), 1629, 1492, 1456, 1407, 1366, 1303, 1222,1146, 987 cm⁻¹. Anal. Calcd for C₁₄H₁₉NO₅S: C, 53.66; H, 6.11; N, 4.47;S, 10.23. Found, C, 53.56; H, 6.25; N, 4.39; S, 10.35. The mother liquorwas concentrated at reduced pressure to afford the sulfoxide 39b as acolorless gum (1.00 g, 76%). [α]_(D) ²⁵ −53.2° (c 1.0, MeOH). ¹H NMR(300 MHz, CDCl₃) (rotamers) [(D6)DMSO) 7.37-7.29 (m, 5H), 5.10-5.03 (m,2H), 4.63-4.56 (m, 1H), 2.83-2.70 (m, 4H), 2.59-2.49 (m, 4H), 2.25-2.20(m, 1H), 2.10-1.97 (m, 1H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ171.87,156.12, 155.65, 136.81, 128.44, 128.37, 127.81, 127.38, 66.50, 57.94,57.76, 49.68, 49.48, 37.83, 31.62, 31.15, 21.54, 21.23. IR (NaCl)ν3500-3200 (COOH), 3063 and 3023 (CH, aromatic), 3000-2800 (CH,saturated), 1700 (CO, acid), 1550, 1455, 1404, 1317, 1222, 1169, 1132,1001, 823, 742, 693 cm⁻¹. HRMS calcd for C₁₄H₁₉NO₅S (M⁺) 314.1062 found314.1074.

N-Benzyloxycarbonyl-N-methyl-L-methionine (40)⁵⁷

To a solution of the sulfoxides (38) (1.3 g, 4.2 mmol) in chloroform (22ml) was added triethylsilane (2.0 ml) and trifluoroacetic acid (22 ml).The reaction mixture was stirred at room temperature for 2 d. Thesolution was then cooled to 0° C. and ammonium iodide (3.02 g) anddimethylsulfide (1.53 ml) were added. The reaction mixture was stirredvigorously for 1 h at 0° C. and then it was diluted with toluene andevaporated at reduced pressure. The residue was taken up in ether andextracted with sodium carbonate solution (10% w/v, 4×15 ml). Thecombined aqueous extracts were washed with ether and then acidified topH 2 with 5 M hydrochloric acid. The aqueous layer was then extractedwith dichloromethane (3×20 ml) and the combined organic extracts werewashed with 5% sodium thiosulfate solution, dried (MgSO₄), filtered andconcentrated in vacuo to give the methionine (40) as a clear colourlessoil (1.01 g, 81%). For analytical purposes this material can be taken upin ether and treated with dicyclohexylamine (1 eq.) to give the DCHAsalt. Mp 97-99° C. [α]_(D) ²³ −17.0° (c 1.0, CHCl₃). ¹H NMR (300 MHz,CDCl₃) (rotamers) 9.47 (brs, 2H), 7.34-7.22 (m, 5H), 5.15-4.95 (m, 2H),4.59-4.51 (m, 1H), 2.88-2.82 (m, 5H), 2.53-2.22 (m, 3H), 2.03 and 1.99(2×s, 3H), 1.90-1.02 (m, 21H). ¹³C NMR (75 MHz, CDCl₃) (rotamers)δ174.68, 174.60, 156.83, 136.96, 136.83, 128.18, 127.56, 127.38, 66.78,66.65, 60.44, 60.16, 52.29, 31.56, 30.54, 30.15, 29.70, 29.66, 28.94,28.69, 25.07, 24.61, 15.40. IR (KBr) ν 3037 (CH, aromatic), 3000-2800(CH, saturated), 2525 and 2452 (NH₂ ⁺), 1702 (CO, carbamate), 1631 (CO₂⁻), 1546, 1517, 1483, 1440, 1390, 1320, 1268, 1170, 1121, 1062, 740cm⁻¹. Anal. Calcd for C₂₆H₄₂N₂O₂S: C, 65.24; H, 8.84; N, 5.85. Found, C,65.32; H, 8.54; N, 5.99.

Example 4 Asparagine

Although it has been shown that carbamoylation of the sidechain ofglutamine allows its conversion to N-methyl glutamine²⁸, this protectionstrategy was not possible with asparagine and so an alternative wassought. Tritylation (Trt) of the asparagine amide sidechain was achievedunder acidic conditions (Scheme 16).⁴⁵ Carbamoylation withN-(benzyloxycarbonyloxy)succinimide (BnOCO₂Succ) then gave the precursor(41)⁴⁵, and subsequent oxazolidination afforded (42) (83%). Thesolubility of the asparagine carbamate (41) was not high and a minimalamount of DMF was included in the reaction protocol to improve substratesolubility and reaction yield. Reductive cleavage of the oxazolidinone(42) gave an 86% yield of the desired N-methyl product (43). In thisreaction the N-methyl group forms with concomitant removal of the tritylgroup under the acidic conditions. The low solubility of the N-methylintermediate (43) necessitated workup by concentration of the reactionmixture and column chromatography of the residue rather than the normalaqueous procedure.

(S)-3-Carbonylbenzyloxy-4-(triphenylmethylaminoacetoyl)-oxazolidin-5-one(42)

The carbamate (41) (2.54 g, 5.0 mmol) was dissolved in a minimum of DMF(ca. 2-3 ml). The solution was then added to toluene (120 ml), followedby camphorsulfonic acid (50 mg) and paraformaldehyde (5 g). The mixturewas heated to reflux until the reaction was complete, ca. 2 h (monitoredby TLC, 40% ethyl acetate-hexane). The reaction mixture was concentratedunder reduced pressure and the residue was taken up in ethyl acetate andthe organic layer was washed with saturated aqueous sodium bicarbonatesolution to remove acidic material. The organic layer was dried (MgSO₄),filtered and evaporated in vacuo. The residue was purified by columnchromatography, eluting with 40% ethyl acetate-hexane to afford theoxazolidinone (42) as a foam (2.16 g, 83%). A sample of the foam wasrecrystallised from hot ether-ethyl acetate to give a solid. Mp 122-123°C. [□]_(D) ²³+60.3° (c 1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) 7.36-7.04(m, 20H), 6.77 and 6.53 (2m, 1H), 5.46-4.89 (m, 3H), 4.63-4.20 (m, 2H),3.30-2.92 (m, 2H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) 8171.68, 167.90,152.40, 144.03, 135.33, 128.63, 128.48, 128.27, 127.95, 127.04, 77.83,77.45, 70.95, 67.70, 37.79, 36.82. IR (KBr disk) ν3352 (CONH), 3088,3060, 3031 and 3007 (CH, aromatic), 3000-2800 (CH, saturated), 1797(C═O, oxazolidinone), 1710 (C═O, carbamate), 1685 (C═O, amide), 1519,1494, 1449, 1417, 1360, 1319, 1256, 1210, 1165, 1130, 1055, 755, 721,700 cm⁻¹. Anal. Calcd for C₃₂H₂₈N₂O₅: C, 73.83; H, 5.42; N, 5.38. Found:C, 73.94; H, 5.39; N, 5.24.

N-Benzyloxycarbonyl-N-methyl-L-asparagine (43)⁵⁸

The oxazolidinone (42) (1.0 g, 1.9 mmol) was dissolved in chloroform (12ml) and to this solution was added triethylsilane (1.2 ml) followed bytrifluoroacetic acid (12 ml) and the reaction mixture was left to stirat room temperature for 2 d. The reaction mixture was concentrated invacuo and the residue was chromatographed on silica eluting with90:10:0.5 chloroform-methanol-water. The appropriate fractions werecombined and concentrated under reduced pressure. The residue wastriturated with ether to give the N-methyl asparagine (43) as acolourless solid (458 mg, 86%). Mp 134-136° C. [α]_(D) ²³ −60.8° (c 1.0,MeOH). ¹H NMR (300 MHz, CD₃OD) 7.34-7.25 (m, 5H), 5.11 (s, 2H),4.89-4.82 (m, 1H), 2.97-2.89 (m, 4H), 2.79-2.66 (m, 1H). ¹³C NMR (75MHz, CD₃OD) (rotamers) δ 175.33, 175.11, 173.66, 158.09, 137.96, 137.76,129.52, 129.05, 128.98, 128.68, 68.68, 68.45, 59.11, 58.53, 36.79,36.29, 34.05, 33.95. IR (KBr disk)ν 3500-3200 (CO₂H), 3427 and 3219(CONH₂), 3115, 3092, 3067, 3033 and 3009 (CH, aromatic), 3000-2800 (CH,saturated), 1714 (CO₂H), 1679 (C═O, carbamate), 1590, 1484, 1451, 1403,1370, 1340, 1256, 1228, 1201, 1169, 1011, 773, 739 cm⁻¹. Anal. Calcd forC₁₃H₁₆N₂O₅: C, 55.71; H, 5.75; N, 9.99. Found: C, 55.65; H, 5.83; N,9.93.

Example 5 Arginine and Homoarginine

The guanidine group of arginine presents several problems for theoxazolidinone chemistry. But N-methyl arginine is an attractive targetgiven the key role arginine plays in many enzymic transformations. Thelysine carbamate (44) was readily available and so the sequence inScheme 17 leading to the N-methyl-lysine (53) was investigated as atrial for the preparation of N-methyl homoarginine. Diazotisation of thecarbamate (44) and its decomposition with sodium acetate led to theformation of the acetate (45) as a mixture with the elimination product(46). These compounds were not separated prior to oxazolidination.Oxazolidination of the mixture gave the expected oxazolidinones (47) and(48), which were separated by column chromatography.

Reductive cleavage of the butenyl oxazolidinone (48) gave the expectedN-methyl amino acid (49) (64%). Reductive cleavage of the oxazolidinone(47) afforded the N-methyl compound (50) (82%). Then the acetate groupwas hydrolysed with aqueous base to give the alcohol (51) and thecarboxylic acid was esterified to give the benzyl ester (52). Treatmentof the benzyl ester (52) with triflic anhydride formed the triflateester in situ. Benzylamine was added to the triflate and displacementprovided the fully protected N-methyl lysine (53). The secondary amine(53) was then treated with aminoiminomethanesulfonic acid⁴⁶ but thisfailed to afford the N-methyl homoarginine (54). In addition, reactionwith the triflylguanidine (55)⁴⁷ also failed to give the desiredhomoarginine (56). It was evident the secondary amine was insufficientlynucleophilic for these guanylation reactions. A similar sequence withornithine intermediates also failed for the same reasons.

The reactions depicted in Scheme 18 were then pursued, which offered asynthesis of N-methyl arginine via direct and less demandingtransformations. The glutamic oxazolidinone (57) was converted to thethioester (58) (92%) by DCC coupling with ethanethiol. Reductivecleavage then proceeded smoothly to give the N-methyl amino acid (59)(87%). The carboxylic acid was protected as the methyl ester (60) viadiazomethylation⁴⁸ in quantitative yield. The resulting thioester wasconverted into the aldehyde (61) by treatment with palladium catalyst inthe presence of triethylsilane.⁴⁹ This material was not purified but wassubmitted directly to the next series of reactions for generating thetarget arginine. Reductive amination with ammonium acetate then affordedthe N-methyl ornithine (62). Reaction of this with the guanylatingreagent (55) gave the desired N-methyl arginine (63) in 49% yield fromthe methyl ester (60).

In addition, conversion of the commercial Fmoc L-nitroarginine (64) wasattempted. The oxazolidination reaction did not give the expectedcompound. Electrospray mass spectrometry and NMR analysis indicated theproduct had a molecular weight of 495, which required the presence ofextra methylene groups. It is proposed that the novel heterocycle (66)was prepared (Scheme 19). Similar chemistry on nitroguanidino compoundsin which there is a second nucleophilic reagent, a primary amine,included in the reaction results in intermolecular condensation of theguanidine, the amine and two equivalents of the formaldehyde.⁵⁰ It isproposed in the current reaction that there is no second nucleophilicreactant and so the weakly nucleophilic nitro group is able to intercepta reactive iminium intermediate and form the isolated product.

There are potentially two possible routes that the reaction can take;either to produce initially either (65) or (66) and then (67) or (68).It was shown that the reaction proceeded via (66) from the detailedanalysis of the NMR spectra and comparison with the data expected forstructure (65). Initially the NMR spectra of compound (65) were run inCDCl₃; however, broad peaks in both the ¹H and the ¹³C NMR spectra wereseen as a result of conformational mobility. Thus, in this solvent therewere a number of missing peaks in the 2D spectra. In DMSO at 333K it wasshown that the peaks were sharper and provided satisfactory 2D spectra.

An accurate assignment of all the protons and carbons in the moleculewas obtained using a combination of COSY, DEPT, HSQC and the HMBCexperiments. The complete assignment is presented in Table 1. TABLE 1¹³C and ¹H NMR data of compound (66) at 300 MHz, 333° K in DMSO. CarbonProton Number of J Carbon Shift Shift Protons Multiplicity coupling *277.34 5.30, 5.22 2 dd 19.95, 4.06  4 53.98 4.05 1H t 6.3  5 171.95  1′26.69 1.58-1.39 2H m  2′ 22.17 1.58-1.39 2H m  3′ 44.79 3.24-3.18 2H m 5′ 77.34 4.84 2H s  4′ 153.84 *6′′ 72.99 4.90-4.89 2 d 1.2  1′′′ (8′′′)126.85 7.63-7.61 2H d 7.13  2′′′ (7′′′) 124.59 7.31 2H t 7.34  3′′′(6′′′) 119.73 7.39 2H t 7.31  4′′′ (5′′′) 127.39 7.86-7.84 2H d 7.424a′′′ (4b′′′) 143.41 8a′′′ (9a′′′) 140.62  9′′′ 46.49 4.28 1H t 5.610′′′ 66.57 4.54 2H m 11′′′ 152.44 OH 9.6 1 s*Overlapping signals

The HMBC experimental data were critical in differentiating betweenstructures (65) and (66). Long-range correlations from the (C4′) atδ153.84 to the protons of (H3′) (δ3.24-3.18), (H5′) (δ4.84) and (H6″)(δ4.90-4.89) would be seen in both. The assignment as 66 was determinedfrom the long-range correlation between the (C3′) at (δ44.79) and the(H5′) at δ4.48 of the γ-position of the propyl chain and thehydroxymethyl group, marked B in FIG. 4. This is not possible instructure 65.

The conclusion depends on the accurate assignment of the carbons andprotons for the 1′, 2′ and 3′ positions. Proton assignments for thesepositions were obtained from the mCOSY experiment, and HSQC and HMBCexperiments were used for the carbon assignments.

The reductive cleavage produces a single product that has a molecularweight of 467 (ESMS). The ¹H and ¹³C NMR spectra clearly indicate thepresence of the N-methyl group and a methylene group associated with theoxatriazine. The reduction of the oxazolidinone (66) to the acid (68)shows the disappearance of the H2 proton peaks at δ5.35 and appearanceof the expected NCH₃ at δ2.72 indicating that only the oxazolidinonering is reductively cleaved. It is apparent that thetriethylsilane/trifluoroacetic acid is able to reduce the5-oxazolidinone but not the new heterocyclic ring formed from thenitroguanidine.

Preparation of the lysine derived oxazolidinones (47) and (48)

Deamination via diazotisation of the lysine carbamate (44) (1.01 g, 3.1mmol) according to the method of Hutton⁵⁹ afforded the acetate (48) andthe alkene (46) as a mixture (1.0 g) which, was not purified. The crudeacetate (45) was taken up in benzene (30 ml) and camphorsulfonic acid(35 mg) and paraformaldehyde (3 g) were added. The mixture was heated toreflux for 2 h and then allowed to cool. The mixture was concentrated atreduced pressure and the residue was taken up in ethyl acetate and theorganic layer was washed with saturated aqueous sodium bicarbonatesolution to remove acidic material. The organic layer was dried (MgSO₄),filtered and evaporated in vacuo. The residue was purified by columnchromatography, eluting with 30% ethyl acetate-hexane to afford firstly,the oxazolidinone (46) as a clear colourless oil (113 mg, 13%). [α]_(D)²⁵ +112.6° (c 1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) 7.34-7.29 (m, 5H),5.70 (brs, 1H), 5.49 (brs, 1H), 5.31-4.95 (m, 5H), 4.31 (brs, 1H),2.12-1.59 (m, 4H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ172.10, 152.68,136.36, 135.35, 128.58, 128.50, 128.19, 127.69, 115.79, 77.77, 67.80,67.67, 55.16, 54.96, 54.24, 29.60, 28.43. IR (NaCl) ν3076 and 3034 (CH,aromatic), 3000-2800 (CH, saturated), 1801 (C═O, oxazolidinone), 1716(C═O, carbamate), 1506, 1413, 1357, 1316, 1251, 1164, 1128, 1050, 919,754, 693 cm⁻¹. Anal. Calcd for C₁₅H₁₇NO₄: C, 65.44; H, 6.22; N, 5.09.Found: C, 65.42; H, 6.31; N, 5.07. Further elution gave theoxazolidinone 47 as a colorless oil (478 mg, 46%). [α]_(D) ²⁵ +86.6° (c1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) 7.33 (s, 5H), 5.48 (brs, 1H),5.23-5.09 (m, 3H), 4.27 (t, 1H, J=5.2 Hz), 3.97 (t, 2H, J=6.2 Hz),2.04-1.76 (m, 2H), 1.98 (s, 3H), 1.43-1.32 (m, 4H). ¹³C NMR (75 MHz,CDCl₃) δ172.03, 170.80, 152.78, 135.29, 128.56, 128.49, 128.16, 77.80,67.80, 63.71, 54.64, 30.18, 27.99, 20.79, 20.76. IR (NaCl) ν3067 and3033 (CH, aromatic), 3000-2800 (CH, saturated), 1801 (C═O,oxazolidinone), 1724 (2×C═O), 1506, 1413, 1361, 1318, 1244, 1167, 1131,1047, 755, 696 cm⁻¹. Anal. Calcd for C₁₇H₂₁NO₆: C, 60.89; H, 6.31; N,4.18. Found: C, 60.80; H, 6.41; N, 4.26.

(S)—N-Benzyloxycarbonyl-N-methyl-2-(3-butenyl)-glycine (49)

The oxazolidinone (48) (310 mg, 1.1 mmol) was taken up in chloroform (6ml) and triethylsilane (540 μl) was added followed by trifluoroaceticacid (6 ml) and the mixture was left to stand at room temperature for 2d. The reaction mixture was diluted with toluene and then concentratedin vacuo and the residue was taken up in ether and extracted withaqueous sodium carbonate solution (4×2 ml). The combined aqueousextracts were washed with ether and then acidified to ˜pH 2 with 5 Mhydrochloric acid. The aqueous phase was then extracted withdichloromethane (3×5 ml). The organic phase was dried (MgSO₄), filteredand evaporated to give a yellow oil (230 mg). The oil waschromatographed on silica eluting with 94:5.5:0.5chloroform-methanol-water to provide the N-methyl amino acid (49) as aclear colourless oil (200 mg, 64%). [α]_(D) ²⁴ −16.6° (c 1.0, CHCl₃). ¹HNMR (300 MHz, CDCl₃) (rotamers) 10.02 (brs, 1H), 7.34-7.30 (m, 5H),5.86-5.50 (m, 1H), 5.19-4.63 (m, 5H), 2.89-2.88 (m, 3H), 2.12-1.60 (m,4H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ176.66, 157.14, 156.45, 136.64,136.47, 136.32, 136.15, 128.39, 127.97, 127.80, 127.68, 116.03, 115.85,67.61, 58.06, 57.80, 31.02, 30.63, 30.10, 29.95, 27.96 and 27.69. IR(NaCl). ν3600-3000 (COOH), 3077 and 3038 (CH, aromatic), 3000-2800 (CH,saturated), 1705 (C═O), 1548, 1451, 1402, 1321, 1210, 1153, 1035, 916,854, 740, 692 cm⁻¹. HRMS calcd for C₁₅H₁₉NO₄ (M+H) 278.1392 found278.1384.

(S)—N-Benzyloxycarbonyl-N-methyl-2-(4-acetoxybutanyl)-glycine (50)

The oxazolidinone (47) (3.26 g, 9.7 mmol) was taken up indichloromethane (50 ml) and triethylsilane (5.0 ml) was added followedby trifluoroacetic acid (50 ml) and the mixture was left to stand atroom temperature for 2 d. The reaction mixture was concentrated in vacuoand the residue was taken up in aqueous sodium bicarbonate solution, andwashed with ether. The aqueous was then acidified 5 M hydrochloric acidand extracted with dichloromethane. The organic phase was dried (MgSO₄),filtered and evaporated to give a yellow oil (2.69 g, 82%) which wasused directly in the next step.

(S)—N-Benzyloxycarbonyl-N-methyl-2-(4-hydroxybutanyl)-glycine (51)

The crude acetate (50) (1.67 g, 4.9 mmol) was treated with 1 M sodiumhydroxide solution (10.8 ml) at 0° C. and left to stir at thattemperature for 1.5 h. The solution was then acidified with dilutehydrochloric acid and extracted with chloroform (6×30 ml). The combinedextracts were dried (MgSO₄) and evaporated in vacuo. The residue wastriturated with ether to afford the alcohol (51) as a colourless solid(1.2 g, 83%). Mp 122-124° C. [α]_(D) ²⁴ −22.3° (c 1.0, acetone). 1H NMR[300 MHz, CD₃COCD₃] (rotamers) 7.38-7.31 (m, 5H), 5.14-5.12 (m, 2H),4.76 (dd, 0.5H, J=4.7, 10.9 Hz), 4.65 (dd, 0.5H, J=4.7, 10.7 Hz), 3.55(t, 2H, J=4.9 Hz), 2.89-2.87 (m, 3H), 1.95-1.35 (m, 6H). ¹³C NMR (75MHz, CDCl₃) (rotamers) δ172.99, 157.51, 156.85, 138.06, 129.18, 128.56,128.34, 67.49, 62.14, 59.03, 32.94, 31.05, 30.57, 29.45, 29.06, 23.37.IR (KBr disk). ν3600-3200 (CO₂H and OH), 3095 and 3030 (CH, aromatic),3000-2800 (CH, saturated), 1738 (C═O, acid), 1650 (C═O, carbamate),1490, 1405, 1322, 1258, 1206, 1162, 1101, 1024, 763 cm⁻¹. Anal. Calcdfor C₁₅H₂₁NO₅: C, 61.00; H, 7.17; N. 4.74. Found: C, 60.87; H, 7.34; N,4.65.

(S)—N-Benzyloxycarbonyl-N-methyl-2-(4-hydroxybutanyl)-glycine benzylester (52)

The acid (51) (300 mg, 1.0 mmol) was dissolved in dimethylformamide (10ml). Anhydrous potassium carbonate (210 mg) was added and the mixturewas vigorously stirred while benzyl bromide (121 μl) was added. Theresulting mixture was stirred at room temperature under a nitrogenatmosphere overnight. It was then diluted with water (150 ml) andextracted with ethyl acetate (3×20 ml) and the combined extracts weredried (MgSO₄) filtered and evaporated at reduced pressure to give thebenzyl ester (52) as a clear gum (334 mg, 87%). A sample was furtherpurified by column chromatography eluting with 30% ethyl acetate-hexaneto give the pure ester (52). [α]_(D) ²⁵ −23.4° (c 1.0, CHCl₃). ¹H NMR(300 MHz, CDCl₃) (rotamers) 7.32-7.24 (m, 10H), 5.18-5.08 (m, 4H), 4.85(dd, 0.5H, J=4.9, 10.8 Hz), 4.62 (dd, 0.5H, J=4.9, 10.5 Hz), 3.59-3.51(m, 2H), 2.86-2.83 (m, 3H), 2.04-1.30 (m, 6H). ¹³C NMR (75 MHz, CDCl₃)(rotamers) δ171.34, 171.38, 156.99, 156.23, 136.46, 136.33, 135.49,135.38, 128.45, 128.35, 128.16, 127.94, 127.79, 127.58, 67.33, 66.67,62.22, 58.64, 58.36, 31.85, 30.87, 30.21, 28.57, 28.18, 22.24, 22.16. IR(NaCl) ν3600-3200 (OH), 3094, 3065 and 3036 (CH, aromatic), 3000-2800(CH, saturated), 1739 (C═O, ester), 1699 (C═O, carbamate), 1456, 1401,1320, 1257, 1212, 1151, 1069, 909, 742, 693 cm⁻¹. Anal. Calcd forC₂₂H₂₇NO₅: C, 68.55; H, 7.06; N, 3.63. Found: C, 68.28; H, 7.24; N,3.72.

N^(α)-Benzyloxycarbonyl-N^(α)-methyl-N^(ε)-benzyl-L-lysine benzyl ester(53)

The alcohol (52) (740 mg, 1.9 mmol) was dissolved in dry dichloromethane(9 ml) and the solution was cooled to −50°. Triethylamine (460 μl) wasadded followed by trifluoromethanesulfonic anhydride (490 μl). After 15min at −50° TLC analysis indicated complete conversion to thecorresponding triflate. Benzylamine (0.82 ml) was then added in oneportion at −50° and the reaction mixture was stirred at this temperaturefor 30 min and then at room temperature overnight. The reaction mixturewas diluted with ether (100 ml) and the organic phase was washed withwater (3×300 ml). The organic phase was dried (MgSO₄), filtered andconcentrated at reduced pressure. The crude residue was purified bycolumn chromatography eluting firstly, with 60% ethyl acetate-hexane andthen 8% methanol-ethyl acetate to afford the lysine (53) as a clear oil(701 mg, 78%).

(S)-3-Carbonylbenzyloxy-4-(2-ethylsulfanylcarbonyl-ethyl)-oxazolidin-5-one(58)

To a sample of the glutamic acid oxazolidinone (57) (2.0 g, 6.8 mmol) indichoromethane (8 ml) was added ethanethiol (1.01 ml, 13.6 mmol) andDMAP (20 mg) and the solution was cooled to 0° C. DCC (1.69 g, 8.2 mmol)was added in one portion and the reaction mixture was stirred at 0° C.for 30 min. Acetic acid (0.8 ml) was then added and stirring wascontinued for 10 min. The mixture was diluted with ether (50 ml) andsuction filtered. The filtrate was washed sequentially with 10% sodiumcarbonate solution (2×20 ml), water, 0.5M hydrochloric acid (20 ml),water and brine. The ethereal solution was then dried (MgSO₄), filteredand concentrated in vacuo to give the thioester (58) as an oil (2.13 g,92%). A sample was further purified for analytical purposes by column 20chromatography on silica eluting with 50% ether-hexane. [α]_(D) ²⁵+99.2° (c 1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) 7.30 (s, 5H), 5.42 (brs,1H), 5.14 (d, 1H, J=4.5 Hz), 5.12 (s, 2H), 4.27 (t, 1H, J=5.7 Hz), 2.79(q, 2H, J=7.4 Hz, 2H), 2.65-2.49 and 2.36-2.11 (2m, 4H), 1.16 (t, 3H,J=7.4 Hz). ¹³C NMR (75 MHz, CDCl₃) δ197.34, 171.36, 152.64, 135.12,128.39, 128.29, 128.04, 77.59, 67.72, 53.71, 38.49, 25.95, 23.10, 14.44.IR (NaCl) ν3097, 3063 and 3033 (CH, aromatic), 3000-2800 (CH,saturated), 1800 (C═O, oxazolidinone), 1718 (C═O, carbamate andthioester), 1500, 1412, 1356, 1317, 1252, 1169, 1131, 1052, 998, 840,756, 696 cm⁻¹. Anal. Calcd for C₁₆H₁₉NO₅S: C, 56.96; H, 5.68; N, 4.15.Found: C, 56.72; H, 5.57; N, 4.30.

(S)-2-(Benzyloxycarbonyl-methyl-amino)-4-ethylsulfanylcarbonyl-butyricacid (59)

A sample of the oxazolidinone (58) (1.0 g, 0.3 mmol) was dissolved indichloromethane (15 ml) and triethylsilane (1.4 ml) was added followedby trifluoroacetic acid (15 ml) and the mixture was left to stand for 3d at room temperature. The solution was then taken up in toluene (50 ml)and evaporated to dryness under reduced pressure. The residue was thentaken up in ether and extracted with 10% sodium carbonate solution. Theaqueous layer was washed with ether and then acidified to ˜pH 2 with 5Mhydrochloric acid. The aqueous phase was then extracted withdichloromethane (4×20 ml). The combined extracts were dried (MgSO₄),filtered and evaporated in vacuo. The residual oil (920 mg) slowlycrystallised. The oil was recrystallised from ether-hexane to afford thecarboxylic acid (59) as a colourless solid (680 mg, 87%). Mp 94-96° C.[α]_(D) ²⁴ −15.6° (c 1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃) 10.22 (brs,1H), 7.32-7.27 (m, 5H), 5.14 (s, 2H), 4.85-4.56 (m, 1H), 2.88-2.81 (m.5H), 2.65-2.05 (m, 4H), 1.21 (t, 3H, J=7.4 Hz). ¹³C NMR (75 MHz, CDCl₃)(rotamers) δ8198.17, 175.29, 157.11, 156.27, 136.36, 128.50, 128.08,127.83, 67.84, 58.46, 40.31, 31.47, 24.49, 23.37, 14.57. IR (KBr disk).ν3700-3200 (CO₂H), 3134, 3097, 3069 and 3033 (CH, aromatic), 3000-2800(CH, saturated), 1736, 1687 and 1648 (3×C═O), 1492, 1455, 1411, 1374,1325, 1254, 1222, 1174, 1096, 1069, 1017, 989, 767, 739 cm⁻¹. Anal.Calcd for C₁₆H₂₁NO₅S: C, 56.62; H, 6.24; N, 4.13. Found: C, 56.75; H,6.30; N, 4.29.

(S)-2-(Benzyloxycarbonyl-methyl-amino)-4-ethylsulfanylcarbonyl-butyricacid methyl ester (60)

The title compound (60) was prepared by diazomethylation of thecarboxylic acid (59) by the standard method.⁴⁸ The methyl ester (60) wasisolated in 100% yield. [α]_(D) ²⁴ −21.8° (c 2.0, CHCl₃). ¹H NMR (300MHz, CDCl₃) (rotamers) 7.26-7.17 (m, 5H), 5.06-5.04 (m, 2H), 4.67 and4.49 (2dd, 1H, J=5.0, 10.5 Hz), 3.59-3.52 (m, 3H), 2.77-2.71 (m, 5H),2.58-2.40 and 2.31-1.91 (2m, 4H), 1.12 (t, 3H, J=7.4 Hz). ¹³C NMR (75MHz, CDCl₃) (rotamers) δ197.68, 170.71, 170.59, 156.42, 155.65, 136.18,136.03, 128.08, 127.63, 127.49, 127.34, 67.09, 57.90, 51.79, 39.96,39.68, 31.32, 30.57, 24.29, 23.99, 22.92, 14.34. IR (NaCl). ν3095, 3063and 3029 (CH, aromatic), 3000-2800 (CH, saturated), 1743 and 1700(3×C═O), 1448, 1403, 1316, 1219, 1180, 1141, 1057, 1007, 907, 742, 695.Anal. Calcd for C₁₇H₂₃NO₅S: C, 57.77; H, 6.56; N, 3.96. Found: C, 58.05;H, 6.74; N, 4.15.

(S)-2-(Benzyloxycarbonyl-methyl-amino)-5-[tert-butoxycarbonylamino-(tert-butoxycarbonylimino)methyl)-pentanoicacid methyl ester (63)

To a sample of the thioester (60) (200 mg, 0.56 mmol) in acetone (1.0ml) was added triethylsilane (300 μl) followed by 10%Palladium-on-charcoal catalyst (50 mg) and the reaction mixture wasstirred vigorously for 1 h. The mixture was filtered through celite andthe filtrate was concentrated under reduced pressure. The residuealdehyde (61) was purified by chromatography on a short silica columneluting with 20% ethyl acetate-hexane to remove the triethylsilane. Thefractions collected were concentrated in vacuo and the residue was takenup in methanol (4 ml) and ammonium acetate (222 mg) was added followedby sodium cyanoborohydride (71 mg) and the mixture was stirred at roomtemperature for 30 min. The solution was concentrated at reducedpressure to about 1 ml and it was then diluted with saturated aqueoussodium bicarbonate solution (10 ml). The aqueous phase was thenextracted with dichloromethane (3×5 ml). The combined extracts weredried (MgSO₄), filtered and concentrated in vacuo to give the primaryamine (62). The amine (62) was taken up in chloroform (filtered throughneutral alumina, 2 ml) and di-boc-triflylguanidine (221 mg, 0.56 mmol)was added followed by diisopropylethylamine (0.15 ml, 0.85 mmol) and themixture was stirred at room temperature for 2 h. The solution wasconcentrated under reduced pressure and the residue was purified bychromatography on silica eluting with chloroform. The material isolatedwas further purified by chromatography eluting with 50% ether-hexane togive the protected N-methyl arginine (63) as a clear colourless oil (149mg, 49%). [α]_(D) ¹⁹ −13.0° (c 0.5, CHCl₃). ¹H NMR (300 MHz, CDCl₃)(rotamers) 11.45 (brs, 1H), 8.28 (brs, 1H), 7.31-7.23 (m, 5H), 5.12 (d,1H, J_(AB)=12.3 Hz), 5.07 (d, 1H, J_(AB)=12.3 Hz), 4.76 and 4.57 (2dd,1H, J=4.8, 10.5 Hz), 3.65-3.58 (m, 3H), 3.43-3.33 (m, 2H), 2.82 (s, 3H),2.02-1.37 (m, 22H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ171.53, 171.35,163.30, 156.81, 156.00, 153.12, 136.41, 136.31, 128.84, 128.32, 127.84,127.56, 82.98, 79.07, 67.34, 58.37, 58.19, 51.99, 40.17, 39.97, 31.07,30.26, 28.12, 27.89, 26.12, 25.84, 25.73. IR (NaCl). ν3335 and 3290(sh),2×NH, 3133, 3104, 3076 and 3033 (CH, aromatic), 3000-2800 (CH,saturated), 1712 and 1633 (4×C═O), 1574, 1446, 1411, 1364, 1327, 1238,1229, 1141, 1053, 866, 809, 762, 746 cm⁻¹. Anal. Calcd for C₂₆H₄₀N₄O₈:C, 58.19; H, 7.51; N, 10.44. Found: C, 58.32; H, 7.56; N, 10.22.

4S-}4-(4-[4-Hydroxymethylimino-2-oxy-4H-(1,2,3,5)-oxatriazin-5-yl]-propyl}-oxazolidin-5-one-3-carboxylicacid 9H-fluoren-9-ylmethyl ester (66)

The nitroarginine carbamate (64) (1.0 g, 2.3 mmol) was dissolved intoluene (50 ml) in a round-bottomed flask fitted for reflux. To thesolution was added camphorsulfonic acid (10 mg) and paraformaldehyde(1.5 g) and the mixture was heated to reflux for 1.5 h. The reactionmixture was cooled and the solvent was decanted from residual solidmaterial. The solvent was concentrated in vacuo and the residue waspurified by column chromatography eluting with 80% ethylacetate-dichloromethane to afford the oxazolidinone (66) as a colourlessfoam (750 mg, 67%). [α]_(D) ³³ +117.8° (c 1.0, CH₂Cl₂). 1H NMR [300 MHz,(D6)DMSO, 298K] 9.76 (s, 1H), 7.93-7.34 (m, 8H), 5.35 (s, 2H), 4.94-4.93(m, 4H), 4.52 (brs, 2H), 4.32 (t, 1H, J=5.4 Hz), 3.57-3.44 (m, 2H), 3.27(s, 1H), 1.90-1.25 (brs, 4H). ¹H NMR [300 MHz, (D₆)DMSO, 333K] 9.60 (s,1H), 7.86-7.31 (m, 8H), 5.26 (dd, 2H, J=20.0, 4.1 Hz), 4.90 (d, 2H,J=1.2 Hz), 4.84 (s, 2H), 4.54 (m, 2H), 4.28 (t, 1H, J=5.6 Hz), 4.04 (t,J=6.4 Hz), 3.24-3.18 (m, 2H), 1.58-1.39 (m, 4H). ¹³C NMR [75 MHz,(D₆)DMSO, 298K] δ172.45, 153.74, 152.75, 143.67, 143.59, 140.87, 127.73,127.21, 124.98, 120.14, 77.75, 77.55, 73.26, 66.82, 54.33, 46.61, 44.98,26.87, 22.38. ¹³C NMR [75 MHz, (D₆)DMSO, 333K] δ171.95, 153.84, 152.44,143.41 and 143.32, 140.62, 127.39, 126.85, 124.59, 124.56, 119.73,77.34, 72.99, 66.57, 53.98, 46.49, 44.79, 26.69, 22.17. IR (KBr disk)ν3289 (OH), 3066, 3041, 3015 and 3007 (CH, aromatic), 3000-2800 (CH,saturated), 1798 (C═O, oxazolidinone), 1713 (C═O, carbamate), 1588,1557, 1412, 1346, 1196, 1136, 1048, 940, 742, 709 cm⁻¹l. HRMS calcd forC₂₄H₂₆N₅O₇ (M+H) 498.1842 found 496.1816.

2S-2-[(9H-Fluoren-9-ylmethylmethoxycarbonyl)-methyl-amino]-5-[hydroxymethyl-(2-oxy-6H-[1,2,3,5]oxatriazin-4-yl-amino-pentanoicacid (68)

The oxazolidinone (66) (100 mg, 0.2 mmol) was dissolved indichloromethane (4 ml) and triethylsilane (0.3 ml) was added followed bytrifluoroacetic acid (4 ml) and the reaction mixture was stirred under anitrogen atmosphere overnight. The mixture was concentrated at reducedpressure. The residue was purified by column chromatography eluting with10% methanol-dichloromethane to afford the N-methyl compound (68) as acolourless foam (60 mg, 60%). [α]_(D) ²² −12.9° (c 1.0, CH₂Cl₂). ¹H MMR[300 MHz, (D₆)DMSO, 300K] 9.60 (s, 1H), 7.84-7.26 (m, 8H), 4.90 (s, 2H),4.88 (s, 2H), 4.35-3.97 (m, 4H), 3.30 (s, 2H), 2.72 (s, 3H), 1.70-1.40(m, 4H). ¹³C NMR [75 MHz, (D₆)DMSO, 300K] δ171.97, 155.63, 153.90,143.64 and 143.59, 140.54, 127.30, 126.76, 124.65, 119.70, 77.41, 73.04,66.56, 57.94, 46.62, 44.95, 30.16, 25.10, 24.05. IR (KBr disk) ν3700-2700 (COOH), 3300-3200 (═NH), 3064, 3039, 3018 and 3009 (CH,aromatic), 1739 (C═O), 1696 (C═O, carbamate), 1589, 1555, 1451, 1409,1315, 1263, 1195, 1158, 1131, 1028, 992, 760, 741 cm⁻¹. HRMS calcd forC₂₄H₂₈N₅O₇ (M+H) 498.1989 found 498.1969.

(S)-3-Carbonylbenzyloxy-4-(1-formyl-1H-indol-3-ylmethyl)-oxazolidin-5-one(72)

A mixture of the tryptophan carbamate (71) (3.0 g, 8.2 mmol), benzene(200 ml), camphorsulfonic acid (100 mg) and paraformaldehyde (5 g) washeated to reflux for 1.5 h. The reaction mixture was concentrated underreduced pressure and the residue was taken up in ether. The ethereallayer was washed with saturated aqueous sodium bicarbonate solution,dried (MgSO₄), filtered and concentrated in vacuo to give an oil. Theoil was further purified by column chromatography, eluting with 60%ether-hexane to give the oxazolidinone (72) as a colourless foam (2.67g, 86%). [α]_(D) ²⁵ +154.0° (c 1.0, CHCl₃). ¹H NMR (300 MHz, CDCl₃)9.31, 8.89 and 8.33-8.31 (2×brs and m, 2H), 7.58-7.04 (m, 9H), 5.21(brs, 3H), 4.59-4.46 (m, 2H), 3.57-3.22 (m, 2H). ¹³C NMR (75 MHz, CDCl₃)δ171.73, 159.13, 155.53, 152.36, 135.25, 134.07, 130.74, 128.65, 125.40,124.67, 124.27, 120.91, 119.68, 118.70, 116.69, 115.91, 109.49, 77.81,67.86, 55.64, 26.11, 25.06. IR (KBr disk). ν 3100 and 3063 (CH,aromatic), 3000-2800 (CH, saturated), 1801 (C═O, oxazolidinone), 1712(C═O, carbamate), 1604, 1459, 1417, 1370, 1241, 1198, 1163, 1127, 1047,1001, 753, 696 cm⁻¹. Anal. Calcd for C₂₁H₁₈N₂O₅: C, 66.66; H, 4.79; N,7.40. Found: C, 66.87; H, 5.06; N, 7.50.

Example 6 Tryptophan

Attempted oxazolidination of the carbamate of tryptophan results indecomposition. This is presumably due to side reactions of the indolenitrogen. An electron-withdrawing protecting group was anticipated tosolve this problem and accordingly, the N-formyl tryptophan (70)⁵¹(Scheme 20) was prepared in quantitative yield from L-tryptophan (69).Carbamoylation then gave the precursor (71) for oxazolidination. Theoxazolidination proceeded in good yield (86%) and the oxazolidinone (72)was isolated as an oil. The following reductive cleavage did not proceedas planned. In all cases two products were isolated. The minor productwas the expected N-methyl tryptophan (73). The major product was theβ-carboline (74). The β-carboline arises by reaction of the intermediateiminium ion with the indole in an intramolecular electrophilic aromaticsubstitution. The resulting carboxylic acid (74) was isolated as itstert-butylammonium salt (75).

To further substantiate the role of the indole, the electrophilicaromatic substitution can be eliminated by reducing the pyrrole ringdouble bond. Accordingly, tryptophan (69) was converted todihydrotryptophan (Scheme 21).⁵² This material underwentbis-carbamoylation to give the precursor (76). Oxazolidination proceededsmoothly to afford the mixture of diastereoisomers (77). The keyreductive cleavage proceeded as expected to afford the N-methyldihydrotryptophan (78) in 83% yield.

(S)—N-Carbonylbenzyloxy-N-methyl-N′-formyl-L-tryptophan (73) and(S)-2-Carbonylbenzyloxy-9-formyl-1,3,4,9-tetrahydro-β-carboline-3-carboxylicacid (74)

To a mixture of the oxazolidinone (72) (500 mg, 1.3 mmol), chloroform (8ml) and triethylsilane (0.6 ml) was added trifluoroacetic acid (8 ml)and the whole was left to stand at room temperature for 2 d. The mixturewas then concentrated at reduced pressure and the residue was taken upin ether. The ethereal solution was extracted with saturated aqueoussodium bicarbonate solution (3×10 ml). The combined aqueous extractswere acidified with dilute hydrochloric acid and extracted withdichloromethane (3×20 ml). The extracts were dried (MgSO₄), filtered andevaporated at reduced pressure. The residue was purified by columnchromatography eluting with 95:5:0.5:0.2chloroform:methanol:water:acetic acid to give firstly, the β-carboline(74) as an oil (340 mg, 69%). The β-carboline can be converted to thetert-butylammonium salt (75) by taking it up in ether and adding anequivalent′of tert-butylamine. The precipitated tert-butylammonium salt(75) can be recrystallised from hot methanol. Mp 162-165° C. [α]_(D) ²⁴+41.3° (c 1.0, MeOH). ¹H NMR [300 MHz, (D₆)DMSO] 9.68, 9.32 and8.21-7.93 (2×brs and m, 4H), 7.48-7.23 (m, 9H), 5.17-4.71 (m, 5H),3.44-3.39 (m, 1H), 2.78-2.72 (m, 1H), 1.06 (s, 9H). ¹³C NMR (75 MHz,CDCl₃) (rotamers) δ172.56, 159.08, 155.94, 155.77, 152.97, 137.14,136.06, 135.27, 130.34, 128.33, 127.70, 127.62, 127.42, 127.18, 123.75,118.41, 114.77, 110.76, 66.07, 54.19, 50.06, 42.12, 27.19, 23.44, 23.21.IR (KBr disk). ν 3000-2800 (CH, saturated), 2743, 2636 and 2554 (NH₃ ⁺),1711 (C═O, carbamate), 1637 (CO₂ ⁻), 1568, 1422, 1386, 1358, 1301, 1222,1102, 1066, 748, 697 cm⁻¹. Anal. Calcd for C₂₅H₂₉N₃O₅: C, 66.50; H,6.47; N, 9.31. Found: C, 66.67; H, 6.54; N, 9.20. Further elutionafforded the N-methyl tryptophan 73 as a solid (110 mg, 22%)., Mp129-130° C. [α]_(D) ²⁵ −49.6° (c 0.5, CHCl₃). ¹H NMR (300 MHz, CDCl₃)(rotamers) 9.35, 8.83 and 8.38-8.36 (2×brs and m, 2H), 7.63-6.94 (m,9H), 5.14-5.01 (m, 3H), 3.50-3.09 (m, 2H), 2.89-2.83 (m, 3H). ¹³C NMR(75 MHz, CDCl₃) (rotamers) δ175.25, 159.41, 156.88, 155.94, 136.29,135.92, 134.33, 130.96, 128.52, 128.19, 127.79, 125.55, 124.89, 124.67,124.21, 122.75, 119.75, 118.58, 116.26, 109.71, 67.83, 67.71, 58.66,58.39, 31.97, 31.81, 24.68, 24.16. IR (KBr disk). ν 3600-3200 (CO₂H),3091 and 3056 (CH, aromatic), 3000-2800 (CH, saturated), 1749 (C═O,acid), 1675 (CO, carbamate), 1605, 1459, 1392, 1319, 1251, 1191, 1135,983, 795, 755, 699 cm⁻¹. Anal. Calcd for C₂₁H₂₀N₂O₅: C, 66.31; H, 5.30;N, 7.36. Found: C, 66.20; H, 5.39; N, 7.16.

(S)-3-Carbonylbenzyloxy-4-[1-carbonylbenzyloxy-2,3-dihydroindol-3(R,S)-ylmethyl]-oxazolidin-5-one(77)

The dihydrotryptophan (76)²⁹ (2.0 g, 4.2 mmol) was dissolved in toluene(100 ml) and the solution was treated with camphorsulfonic acid (60 mg)and paraformaldehyde (5 g) and heated at reflux for 1 h. The clearsolution was concentrated in vacuo and the residue was taken up in ethylacetate and washed with saturated aqueous sodium bicarbonate solution.The organic layer was dried (MgSO₄), filtered and evaporated at reducedpressure to give a tan coloured oil (1.56 g). The oil was purified bycolumn chromatography eluting with 20% ethyl acetate-hexane to give theoxazolidinone (77) as a colourless oil (1.38 g, 68%). ¹H NMR (300 MHz,CDCl₃) 7.89-6.93 (m, 14H), 5.53 (brs, 1H), 5.26-5.09 (m, 5H), 4.22-4.18and 3.78-3.35 (2×m, 4H), 2.31-2.12 (m, 2H). ¹³C NMR (75 MHz, CDCl₃)8171.71, 153.18, 152.78, 142.01, 136.16, 135.01, 132.87, 128.65, 128.49,128.32, 128.12, 127.98, 123.86, 122.70, 114.84, 77.63, 68.21, 68.11,66.92, 53.56, 53.16, 36.76, 36.03, 35.66. IR (NaCl). ν3000-2800 (CH,saturated), 1798 (C═O, oxazolidinone), 1712 (CO, carbamate), 1599, 1457,1412, 1347, 1261, 1140, 1032, 752 cm⁻¹. Anal. Calcd for C₂₈H₂₆N₂O₆: C,69.12; H, 5.39; N, 5.76. Found: C, 69.37; H, 5.67; N, 5.57.

N,N′-bis-Carbonylbenzyloxy-3(R,S)-3-[2(S)-2-carboxy-2-methylamino-ethyl]-N-methyl-2,3-dihydroindole(78)

To a solution of the dihydrotryptophan oxazolidinone (77) (1.2 g, 2.5mmol) in chloroform (13 ml) was added triethylsilane (1.2 ml) andtrifluoroacetic acid (13 ml). The mixture was left to stand for 2 d andit was then diluted with toluene and concentrated under reducedpressure. The greenish residue was chromatographed on a short silica gelcolumn eluting with chloroform-methanol-water 93:6.5:0.5. Theappropriate fractions were collected and the solvent was removed invacuo. The residue was further purified by chromatography eluting withthe same solvent system to give the N-methyl dihydrotryptophan (78) as aclear pale yellow oil (1.0 g, 83%). ¹H NMR (300 MHz, CDCl₃) 7.70-6.94(m, 14H), 5.26-5.11 (m, 4H), 5.00-4.90 and 4.77-4.69 (2×m, 1H),4.18-3.96 and 3.79-3.22 (2×m, 3H), 2.95-2.88 (m, 3H), 2.42-1.95 (m, 2H).¹³C NMR (75 MHz, CDCl₃) δ174.75, 174.48, 157.03, 156.21, 152.87, 141.84,136.16, 135.87, 133.36, 128.49, 128.43, 128.14, 128.03, 127.67, 124.38,123.52, 122.78, 114.96, 67.77, 67.02, 57.03, 56.74, 53.21, 36.28, 34.54,34.06, 31.07. IR (NaCl). ν3500-3200 (CO₂H), 3064 and 3038 (CH,aromatic), 3000-2800 (CH, saturated), 1703 (C═O), 1600, 1487, 1456,1411, 1321, 1214, 1146, 1089, 1035, 971, 911, 856, 746, 697 cm⁻¹. HRMScalcd for C₂₈H₂₈N₂O₆ (M⁺) 488.1947 found 488.1944.

Example 7 Histidine

Again the basic and highly nucleophilic nature of the histidinesidechain caused problems in the initial attempts to form N-methylhistidine. Selective formation of the α-amino carbamate is difficulttoo. So the following sequence (Scheme 22) was adopted. Histidine methylester hydrochloride salt (79) was carbamoylated with two equivalents of(benzyloxycarbonyloxy)succinimide to give the bis-carbamate (60).Treatment of the bis-carbamate with propylamine effects removal of theimidazole carbamate. The reaction mixture was then evaporated underreduced pressure and the residue in acetonitrile was treated with2,4-dinitrofluorobenzene, which undergoes a nucleophilic aromaticsubstitution to afford the dinitrophenyl (DNP) imidazole (81). Treatmentof this compound with a mixture of acetic acid and 2M hydrochloric acidresulted in hydrolysis of the methyl ester to afford the acid as ahydrochloride salt (82).

This acid (82) is the precursor for the formation of the oxazolidinone,but standard conditions for its formation could not be used due to theinsolubility of (82). This was overcome by dissolving the hydrochloride(82) in acetic acid and acetic anhydride in the presence ofcamphorsulfonic acid catalyst. Treatment of this mixture withparaformaldehyde afforded the required oxazolidinone (83) in high yield(>75%). Reductive cleavage then gave the N-methyl histidine carbamate(84) with the sidechain imidazole still protected with the dinitrophenylgroup.

N,N^(imid)-Biscarbonylbenzyloxy-L-histidine methyl ester (80)

A sample of the methyl ester (79) (1.0 g, 4.1 mmol) in acetonitrile (25ml) was cooled to 0° with vigorous stirring and triethylamine (2.3 ml)was added followed by BnOCO₂-Succ (2.16 g, 8.7 mmol). The reactionmixture was stirred at 0° C. for 30 min and then at room temperatureovernight. The solution was concentrated at reduced pressure and theresidue was taken up in ethyl acetate and washed with water (3×25 ml).The organic phase was dried (MgSO₄), filtered and evaporated in vacuo togive a pale yellow oil (1.57 g). The oil was purified by columnchromatography on silica eluting with 40% ethyl acetate-hexane to givethe carbamate (80) as a colourless oil which slowly crystallised onstanding (1.3 g, 72%). Mp 63-65° C. [α]_(D) ²² +28.0° (c 1.0, CHCl₃). ¹HNMR (300 MHz, CDCl₃) 8.01 (s, 1H), 7.41-7.24 (m, 10 H), 7.17 (s, 1H),6.10 (d, 1H, J=8.1 Hz), 5.35 (s, 2H), 5.07 (s, 2H), 4.67-4.61 (m, 1H),3.68 (s, 3H), 3.12-2.99 (m, 2H). ¹³C NMR (75 MHz, CDCl₃) δ171.68,155.83, 148.13, 138.76, 136.25, 133.78, 136.82, 129.04, 128.71, 128.58,128.29, 127.89, 114.51, 69.74, 66.71, 53.36, 52.25, 29.80. IR (KBrdisk). ν 3175, 3139, 3108 and 3040 (CH, aromatic), 3000-2800 (CH,saturated), 1755 (C═O, ester), 1695 (C═O, carbamate), 1531, 1447, 1409,1257, 1015, 871, 736, 697 cm−₁. Anal. Calcd for C₂₃H₂₃N₃O₆: C, 63.15; H,5.30; N, 9.61. Found: C, 63.35; H, 5.26; N, 9.78.

N-Carbonylbenzyloxy-N^(imid)-(2,4-dinitrophenyl)-L-histidine methylester (81)

The bis-carbamate (80) (1.0 g, 2.3 mmol) was dissolved in propylamine(30 ml) and the solution was left to stir at room temperature for 1 h.The solvent was removed by evaporation at reduced pressure. The residuewas taken up in ethyl acetate (100 ml) and the solution was againconcentrated under reduced pressure. The residue was taken up inacetonitrile (20 ml) and triethylamine (0.64 ml) was added in oneportion followed by 1-fluoro-2,4-dinitrobenzene (336 μl) and thesolution was left to stir in the dark overnight. The solution wasconcentrated in vacuo and the residue was taken up in ethyl acetate andwashed with water (3×50 ml). The organic layer was dried (MgSO₄),filtered and concentrated to provide a crude yellow oil (1.5 g). The oilwas chromatographed on silica eluting with 88:10:2dichloromethane-acetone-methanol to give the methyl ester (81) as ayellow gum (0.9 g, 84%). [α]_(D) ²³ +23.7° (c 1.0, CHCl₃). ¹H NMR (300MHz, CDCl₃) 8.80 (d, 1H, J=2.4 Hz), 8.53 (dd, 1H, J=2.4, 8.7 Hz), 7.76(s, 1H), 7.67 (d, 1H, J=8.7 Hz), 7.31-7.24 (m, 5H), 6.84 (s, 1H), 6.12(d, 1H, J=8.1 Hz), 5.07 (s, 2H), 4.69-4.66 (m, 1H), 3.71 (s, 3H), 3.15(d, 2H, J=3.4 Hz). ¹³C NMR (75 MHz, CDCl₃) δ 171.71, 155.97, 146.98,144.32, 139.15, 134.68, 136.46, 129.35, 128.43, 128.30, 128.05, 121.28,117.44, 66.89, 53.48, 52.54, 29.85. IR (NaCl) ν3109, 3067 and 3021 (CH,aromatic), 3000-2800 (CH, saturated), 1717 (C═O), 1609, 1536 and 1346(NO₂), 1449, 1255, 1214, 1055, 911, 835, 743 cm ⁻¹.

N-Carbonylbenzyloxy-N^(imid)-(2,4-dinitrophenyl)-L-histidinehydrochloride salt (82)⁶¹

The methyl ester (81) (900 mg, 1.9 mmol) was dissolved in a mixture ofglacial acetic acid (10 ml) and 2 M hydrochloric acid (10 ml) and thesolution was left in the dark at room temperature for 3 d. The mixturewas then concentrated at reduced pressure. The residue crystallised onstanding and was purified by recrystallisation from methanol-ether toafford the hydrochloride salt (82) as a pale yellow solid (870 mg, 92%).Mp 169-171° C. [α]_(D) ³⁰ −8.1° (c 1.0, MeOH). ¹H NMR [300 MHz,(D₆)DMSO] 9.48 (s, 1H), 9.00 (s, 1H), 8.81 (d, 1H, J=8.7 Hz), 8.15 (d,1H, J=8.7 Hz), 7.81-7.78 (m, 2H), 7.38-7.27 (m, 5H), 5.03 (d, 1H,J_(AB)=12.5 Hz), 4.99 (d, 1H, J_(AB)=12.6 Hz), 4.46-4.39 (m, 1H),3.28-3.05 (m, 2H). ¹³C NMR [75 MHz, (D₆)DMSO] δ172.20, 156.06, 148.11,143.87, 136.83, 132.99, 131.86, 136.75, 131.52, 129.45, 128.34, 127.87,127.69, 121.53, 120.56, 65.61, 52.90, 26.51. IR (KBr disk) ν3200-2500,(CO₂H), 3112 and 3064 (CH, aromatic), 3000-2800 (CH, saturated), 2604,(⁺NH Cl⁻), 1705 (C═O), 1614, 1542 and 1345 (NO₂), 1447, 1389, 1241,1056, 911, 843, 745, 697, 633 cm⁻¹. Anal. Calcd for C₂₀H₁₈ClN₅O₈: C,48.84; H, 3.69; N, 14.24. Found: C, 48.87; H, 3.83; N, 14.23.

(S)-3-Carbonylbenzyloxy-4-[3H-3-(2,4-dinitrophenyl)-imidazol-4-ylmethyl]-oxazolidin-5-one(83)

To a solution of the carbamate (82) (200 mg, 0.4 mmol) in glacial aceticacid (5 ml) was added camphorsulfonic acid (10 mg), acetic anhydride (50μl) and paraformaldehyde (50 mg). The mixture was heated with stirringat 85° C. for 2.5 h under a nitrogen atmosphere. The mixture was cooledto room temperature and then concentrated at reduced pressure. Theresidue was taken up in ethyl acetate and washed with aqueous sodiumcarbonate solution (10% w/v, 3×20 ml). The ethyl acetate phase was dried(MgSO₄), filtered and evaporated to dryness. The residual material wasfurther purified by column chromatography eluting with ethyl acetate togive the oxazolidinone (83) as a yellow foam (133 mg, 66%). [α]_(D) ²⁵+148.3° (c 1.0, CHCl3). 1H NMR (300 MHz, CDCl3) 8.81 (d, 1H, J=2.5 Hz),8.54 (dd, 1H, J=2.5, 8.7 Hz), 7.66 (d, 1H, J=8.7 Hz), 7.62 (s, 1H),7.32-7.24 (m, 5H), 6.84-6.64 (m, 1H), 5.37 (brs, 1H), 5.26-5.10 (m, 2H),4.89 (d, 1H, J=3.7 Hz), 4.51-4.49 (m, 1H), 3.49-3.37 (m, 1H), 3.18 (dd,1H, J=2.4, 14.9 Hz.). 13C NMR (75 MHz, CDCl₃) δ 172.03, 152.31, 146.95,144.35, 138.27, 135.64, 134.88, 136.69, 129.36, 128.53, 128.39, 128.29,121.24, 117.95, 78.10, 67.69, 54.61, 28.57, 27.77. IR (KBr disk) ν 3110(CH, aromatic), 3000-2800 (CH, saturated), 1800 (C=O, oxazolidinone),1714 (C=O, carbamate), 1611, 1540 and 1352 (NO₂), 1503, 1419, 1253,1132, 1051, 748, 699 cm⁻¹. Anal. Calcd for C₂₁H₁₇N₅O₈: C, 53.96; H,3.67. Found: C, 53.82; H, 3.72.

(S)—N-Carbonylbenzyloxy-N-methyl-N′-(2,4-dinitrophenyl)-L-histidine (84)

To a solution of the oxazolidinone (83) (460 mg, 1.0 mmol) in chloroform(5 ml) was added triethylsilane (470 μl) and trifluoroacetic acid (5 ml)and the reaction mixture was left to stand for 2 d. The solution wasthen concentrated under reduced pressure. The residue was taken up in aminimum of 95% chloroform-methanol and the precipitate, which formed,was filtered off at the pump to give the N-methyl amino acid (84) (225mg). The filtrate was concentrated in vacuo and the residue was purifiedby column chromatography eluting with 92:7.5:0.5chloroform-methanol-water to afford the N-methyl amino acid (84) (150mg). The combined solids were recrystallised from methanol-ether to givethe title compound (84) as a solid (375 mg, 81%). Mp 165-167°C. [α]_(D)²⁵ −24.7° (c 1.0, MeOH). ¹H NMR [300 MHz, (D₆)DMSO] (rotamers) 8.92-8.91(m, 1H), 8.65-8.62 (m, 1H), 7.98 (brs, 1H), 7.92-7.88 (m, 1H), 7.28-7.19(m, 6H), 5.04-4.95 (m, 2H), 4.88-4.79 (m, 1H), 3.13-2.99 (m, 2H),2.82-2.79 (m, 3H). ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ 172.14, 155.80,155.47, 146.22, 143.52, 139.72, 136.85, 134.63, 137.03, 129.36, 128.69,128.32, 127.67, 127.21, 127.11, 121.32, 117.06, 116.94, 66.29, 66.18,58.93, 58.79, 31.83, 31.64, 27.61, 27.14. IR (KBr disk) ν 3600-3200(CO₂H), 3185, 3130 and 3041 (CH, aromatic), 3000-2800 (CH, saturated),1734 (C═O, acid), 1680 (C═O, carbamate), 1618, 1545 and 1347 (CNO₂),1492, 1460, 1402, 1308, 1187, 1143, 1087, 842 cm⁻¹. Anal. Calcd forC₂₁H₁₉N₅O₈: C, 53.73; H, 4.08; N, 14.92. Found: C, 53.55; H, 4.07; N,14.65.

Example 8 Proline

Due to the tertiary substitution of the α-amino nitrogen in N-methylproline there was limited interest in its synthesis as it can not bereadily incorporated in peptide sequences except at the N-terminus. Theformation of the proline oxazolidinone (88) has been reported⁵³ thoughits synthesis is not high yielding. The isolation of the oxazolidinone(85) can be avoided by the simple expedient of combining aqueousformaldehyde and proline (86) in methanol (Scheme 23). This mixture wasthen subjected to hydrogenating conditions to afford the N-methylproline (87) in near quantitative yield. This approach was employed byLin et al.⁵⁴ to prepare an N-methyl proline ester from a proline ester.

N-methyl-L-proline (87)

L-Proline (86) (2.0 g, 17.4 mmol) was dissolved in methanol (20 ml) andto this solution was added 40% aqueous formaldehyde solution (1.4 ml,19.1 mmol). This was followed by the addition of 10%palladium-on-charcoal catalyst (500 mg) and the resulting slurry wasstirred in a hydrogen atmosphere overnight. The slurry was then filteredthrough a Celite pad to remove the catalyst. The pad was washed withmethanol and the combined filtrates were concentrated under reducedpressure. The residue was taken up in ethanol-benzene (1:1, 100 ml) andconcentrated a second time to provide a solid, which was recrystallisedfrom methanol-diethyl ether. In this way N-methyl proline (87) wasisolated as fine needles (2.2 g, 98%). Mp 142-145° C. [α]_(D) ²³ −78.0°(c 2.0, MeOH). ¹H NMR (300 MHz, D₂O) 3.71-3.65 and 3.55-3.51 (2m, 1H),3.00-2.91 (m, 1H), 2.74 (s, 3H), 2.34-2.28 (m, 1H), 1.99-1.78 (m, 3H).¹³C NMR (75 MHz, CDCl₃) δ 173.06, 70.18, 55.83, 40.26, 28.34, 22.37. IR(KBr disk) ν 3000-2800 (CH, saturated), 2675 and 2605 (ammonium ion),1669 (CO₂H), 1612 (CO₂ ⁻), 1468, 1401, 1354, 1327, 1234, 1183, 1112,1056, 1025, 808, 775 cm⁻¹. HRMS calcd for C₆H₁₁NO₂ (M⁺) 129.0790 found129.0784.

Summary of Examples 1 to 8

5-oxazolidinone chemistry has been applied to the 20 common α-aminoacids (and some others) in the formation of N-methyl derivatives, it ispossible to classify the compounds according to their ease ofmanipulation. In the first group are those α-amino acids with sidechainsthat do not interfere with the oxazolidination and subsequent reductivecleavage. Into this group fits glycine, alanine, valine, leucine,isoleucine, phenylalanine, aspartic acid, glutamic acid, proline,tyrosine and phenylglycine. Historically, it is these-amino acids thathave been concentrated on by other workers.⁵³ Methionine gives one ofthe highest yields of the corresponding 5-oxazolidinone but does notreact well in the reductive cleavage. The second category includes thoseα-amino acids for which a simple sidechain protection reaction that isalso compatible with standard solid phase deprotection conditions allowstheir participation in the oxazolidinone chemistry. These amino acidsare serine, threonine, cysteine, tyrosine, lysine, asparagine, glutamineand ornithine. Tyrosine has been included in both categories becausewhile the N-methylation sequence works in moderate to low yield withoutthe phenolic hydroxyl protected, sidechain benzylation substantiallyimproves the yield. The third category is those amino acids that requiredevoted synthetic schemes and more exotic functional group protection.This group currently consists of the problematic α-amino acids arginine,homoarginine, histidine, tryptophan and methionine.

Example 9 Dipeptides

A number of dipeptides that would be suitable for peptide incorporationhave been prepared. They are not the most ideal examples as solid phasepeptide synthesis methods most frequently use Fmoc and Boc protection ofthe N-terminus. The N-methyl amino acid chemistry is compatible withthese groups, but the early development work has been entirely with Cbzprotection on the nitrogen. This Cbz group works well in synthesis forstandard solution phase approaches.

Preparation of N-Carbonylbenzyloxy-L-Leu-L-N-Methyl-Val-tert-Butyl Ester

R²=isopropyl

R=isobutyl

R″=benzyl

The N-methyl valine ester (500 mg, 1.3 mmol) was dissolved in drydichloromethane (4 ml) and stirred under a nitrogen atmosphere at 0° C.The leucine acid (1.2 eq, 1.6 mmol, 421 mg) was added followed by PyBroP(1.2 eq, 1.6 mmol 746 mg) and DIPEA (4.0 eq, 5.3 mmol, 927 μl). Themixture was stirred for 3 h at 0° C. while being monitored by TLC. Themixture was then passed through a celite pad and the cake was washedwith methanol. The filtrate was concentrated at reduced pressure and theresidue was passed through silica gel, eluting with 20% ethylacetate-hexane to give the title dipeptide as a slightly opaque, pungentsmelling residue (576 mg, 91%); [α]²⁵ _(D)=−61.1 (c, 0.99 in CH₂Cl₂);v_(max) (NaCl)/cm⁻¹ 3299 (NH), 2963 (Aryl CH), 2874 (saturated CH),1728, 1648 (C═O), 1528, 1463, 1368, 1252, 1161, 1047, 740, 698; ¹H NMR(300 MHz, CDCl₃) (rotamers) δ 7.34-7.27 (5 H, m, ArH), 5.60-5.28 (1 H,m, NH), 5.08 (2 H, s, PhCH₂), 4.79-4.69 (2 H, m, 2×α-H), 3.03-2.90 (3 H,m, NCH₃), 2.36-2.08 [1 H, m, CH(CH₃)₂], 1.86-1.61 [1 H, m, (CH₃)₂CH],1.44 [9 H, s, (CH₃)₃], 1.07-0.82 (12 H, m, CH₃×4); ¹³C NMR (75 MHz,CDCl₃) (rotamers) δ 173.79, 169.79 and 156.14 (3×C═O), 136.48 (Aryl C),128.39, 127.95, 127.90, 127.81, 127.40 and 126.84 (Aryl CH), 81.37[C(CH₃)₃], 65.76 (PhCH₂), 60.29 (α-C_(Leu)), 49.47 (α-C_(Val)), 42.22(NHCH₂), 31.03 (NCH₃), 27.92 [C(CH₃)₃], 27.06 (NHCH₂CH), 24.51(NCH₃CHCH), 23.32, 21.79, 19.78 and 18.83 (4×CH₃); E.S.M.S. m/z 457(M+Na, 32 ), 435 [M+1, 100%], 391 (6), 379 (29), 361 (6), 262 (4), 214(3).

Preparation of N-Carbonylbenzyloxy-L-Phe-L-N-Methyl-Phe-tert-Butyl Ester

R²=benzyl

R=benzyl

R″=benzyl

The N-methyl phenylalanine ester (350 mg, 0.8 mmol) was dissolved in drydichloromethane (4 ml) and stirred under a nitrogen atmosphere at 0° C.The phenylalanine acid (1.2 eq, 295 mg, 1.0 mmol) was added along withPyBroP (1.2 eq, 460 mg, 1.0 mmol) and DIPEA (4.0 eq, 573 μl, 3.3 mmol).The reaction was stirred for 2 h at 0° C. The mixture was then passedthrough a celite pad and the cake was washed with methanol. The solutionwas concentrated in vacuo and the residue was subjected to flash columnchromatography, elutin with 30% ethyl acetate-hexane to give thedipeptide as a lightly opaque residue (235 mg, 55%); [α]²⁵ _(D)=−49.6(c, 0.45 in CH₂Cl₂); v_(max) (NaCl)/cm⁻¹ 3308 (NH), 2977 (Aryl CH), 2933(saturated CH), 1727 and 1647 (2×C═O), 1497, 1249, 1155, 1048, 740, 698;¹H NMR (300 MHz, CDCl₃) (rotamers) δ 7.34-7.09 (15 H, m, ArH), 5.41-4.60(5 H, m, NH, PhCH₂ and 2×α-H), 3.32-2.30 (7 H, m, NHCHCH₂, NCH₃ andNCH₃CHCH₂), 1.41 [9 H, s, C(CH₃)₃]; ¹³C NMR (75 MHz, CDCl₃) (rotamers) δ171.51, 169.34 and 155.40 (3×C═O), 136.95, 136.18, 136.00 (Aryl C),129.45, 129.30, 129.10, 128.82, 128.38, 128.33, 128.14, 127.97, 127.80,127.67, 127.07, 126.76, 126.53 (Aryl CH), 81.81 [C(CH₃)₃], 66.48(PhCH₂), 62.14 (α-C_(MePhe)), 51.83 (α-C_(Phe)), 38.67 (NCH₃CHCH₂),34.70 (NHCHCH₂), 32.73 (NCH₃), 27.88 [C(CH₃)₃]; E.S.M.S. m/z 539 (M+Na,8%), 517 [M+1, 100%], 461 (22), 443 (6).

Preparation of N-9-Fluorenylmethoxycarbonyl-L-Val-L-N-Methyl-Thr(O-benzyl)-tert-Butyl Ester

R²=CH(OBn)CH₃

R=isopropyl

R″=9-fluorenylmethyl

The N-Fmoc-N-methyl threonine ester (520 mg, 1.03 mmol) was dissolved in33% diethylamine/DMF (6 ml) at room temperature for 1 h. The reactionmixture was concentrated in vacuo to a residue. The residue wasdissolved in dry dichloromethane (6 ml) and stirred under a nitrogenatmosphere at 0° C. The valine acid (1.2 eq, 457 mg) was added alongwith PyBroP (627 mg, 1.3 eq) and DIPEA (3.0 eq, 0.541 ml). The reactionwas left to stir at room temperature overnight. The mixture was dilutedwith ether (30 ml) and was then washed sequentially with dilutehydrochloric acid, saturated sodium bicarbonate solution, brine, dried(MgSO₄), filtered and evaporated to dryness at reduced pressure. Theresidue was subjected to flash column chromatography, eluting with 20%ethyl acetate-hexane to give the dipeptide as a clear colourless oil(510 mg, 82%). A small portion was chromatographed for a second time foranalytical purposes; [α]²⁵ _(D)=+49.6 (c, 1.0 in CHCl₃) ; v_(max)(NaCl)/cm⁻¹ 3302 (NH), 3089 3066, 3038 (Aryl CH), 3000-2800 (saturatedCH), 1727 and 1643 (2×C═O), 1525, 1506, 1499, 1479, 1451, 1369, 1299,1160, 1110, 1088, 1030, 758, 738, 698; ¹H NMR (300 MHz, CDCl₃) δ7.75-7.24 (13 H, m, ArH), 5.81, (1H, d, J=9.2 Hz, NH), 5.43 (1 H, d,J=4.2 Hz, β-CH-Thr), 4.72-4.20 (7 H, m, NCHCO+2, CHCH₂, PhCH₂), 3.30 (3H, s, NCH₃), 1.44-1.39 [9 H, m, C(CH₃)₃]1, 1.17 (3 H, d, J=6.2 Hz, CH₃),1.08 (3 H, d, J=6.6 Hz, CH₃), 1.01 (3 H, d, J=6.6 Hz, CH₃); ¹³C NMR (75MHz, CDCl₃) (rotamers) δ 173.57, 168.16 (2×C═O), 156.18 (C═O), 143.70,143.60, 141.01, 138.14 (Aryl C), 127.97, 127.39, 127.17, 126.95, 126.79,124.89, 124.08, 119.67 (Aryl CH), 81.51 [C(CH₃)₃), 75.02 (CHO), 71.45(CH2), 66.69 (PhCH₂), 60.90, 55.39 (2×Cα), 46.94 (CH), 33.92 (NCH₃),30.99 (NHCHCH), 27.78, 27.56 (tBu), 19.24, 17.41, 15.67 (CH₃×3).

Preparation ofN-9-Fluorenylmethoxycarbonyl-L-Leu-L-N-Methyl-Gly-tert-Butyl Ester

-   R²=H-   R=isobutyl-   R″=9-fluorenylmethyl

Sarcosine tert-butyl ester (500 mg, 3.7 mmol), triethylamine (0.3 ml)and N-Fmoc-leucine (1.4 g, 4 mmol) and PyBrop (1.75 g, 3.7 mmol) wereadded to dichloromethane (16 ml) in a 50 mL round bottomed flask with amagnetic stirring. The reaction mixture was left to stir at roomtemperature for 2.5 hours under an atmosphere of nitrogen. The solutionwas washed with dilute citric acid solution, sodium bicarbonatesolution, brine, dried (MgSO₄), filtered and concentrated under reducedpressure. The crude residue was purified by column chromatography onsilica eluting with 30% ethyl acetate-hexane to produce the dipeptide asa white solid (440 mg, 40%). ¹H NMR (300 MHz, CDCl₃) δ7.67-7.19 (8H, m,ArH), 5.82-5.79 (1H, m, NH), 4.70-4.50 (3H, m, NCH₂ and α-CH), 4.07-3.58(3H, m, CHCH₂), 3.06-2.91 (3H, m, NCH₃), 1.90-0.84 [18H, m, CH₂CH(CH₃)₂and tBu]. ¹³C NMR (75 MHz, CDCl₃) δ173.77, 172.03, 156.09, 143.57,143.37, 140.90, 127.33, 126.71, 124.85, 119.59, 66.72, 49.51, 48.96,46.78, 41.29, 36.17, 24.20, 23.00, 21.27.

Preparation ofN-9-Fluorenylmethoxycarbonyl-L-Phe-L-N-Methyl-Gly-tert-Butyl Ester

-   R²=H-   R=isobutyl-   R″=9-fluorenylmethyl

Sarcosine tert-butyl ester (350 mg, 2.7 mmol), triethylamine (0.25 ml),N-Fmoc phenylalanine (1.0 g, 2.6 mmol) and PyBrop (1.1 g, 2.5 mmol) wereadded to dichloromethane (10 ml) in a 50 mL round bottomed flask with amagnetic stirring. The reaction mixture was left to stir at roomtemperature for 1 hour under an atmosphere of nitrogen. The reaction wasmonitored by tlc. The solution was washed with dilute citric acidsolution, sodium bicarbonate solution, brine, dried (MgSO₄), filteredand concentrated at reduced pressure. The crude residue was purified bycolumn chromatography on silica eluting with 30% ethyl acetate-hexane toproduce the dipeptide as a white solid (600 mg, 61%). ¹H NMR (300 MHz,CDCl₃) δ7.76-7.20 (13H, m, ArH), 5.81-5.78 (1H, m, NH), 5.02-5.00 (1H,m, α-CH Phe), 4.37-4.03 (7H, m, CHCH₂, NCH₂, 2×αCH), 3.11-2.93 (5H, m,NCH₃, CHCH₂), 1.53-1.45 (9H, m, tBu). ¹³C NMR (75 MHz, CDCl₃) δ171.49,167.41, 155.3, 143.58, 140.93, 136.01, 135.73, 129.28, 129.12, 128.06,127.32, 126.71, 126.61, 124.88, 119.59, 81.63, 66.68, 50.06, 38.85,35.89, 27.73, 27.65.

Preparation of N-9-Fluorenylmethoxycarbonyl-L-Pro-L-N-MethylAla-tert-Butyl Ester

-   R²=methyl-   R=CH₂CH₂CH₂ (proline ring)-   R″=9-fluorenylmethyl

N-Methyl alanine tert-butyl ester (360 mg, 2.4 mmol), triethylamine(0.42 ml), N-Fmoc proline (810 mg, 2.4 mmol) and PyBrop (1.2 g, 1.2 eq.)were added to dichloromethane (10 ml) in a 50 mL round bottomed flaskwith magnetic stirring. The reaction mixture was left to stir at roomtemperature for 12 h under an atmosphere of nitrogen. The reaction wasmonitored by tic. The reaction mixture was washed with dilute citricacid solution, sodium bicarbonate solution, brine, dried (MgSO₄),filtered and concentrated at reduced pressure. The crude residue waspurified by column chromatography on silica with 30% ethylacetate-hexane to produce the didpeptide as a white solid (50 mg, 4.4%).¹H NMR (300 MHz, CDCl₃) δ7.76-7.28 (8H, m, ArH), 4.96-4.25 (7H, m, 2×αH,NCH₂, CHCH₂), 3.11 and 2.93 (3H, 2s, NCH₃), 1.61-0.48 (16H, m, tBu,CHCH₃, CH₂CH₂).

Example 10 Preparation of3-Benzyloxycarbonyl-4,4-dimethyl-oxazolidin-5-one (Z-AIB Oxazolidinone)

Z-AIB—OH (1.3 g, 5.5 mmol) was suspended in toluene (50 mL) in a 100 mLround bottomed flask fitted with a condenser. Paraformaldehyde (1.0 g)and a catalytic amount of camphorsulfonic acid were added, and themixture was refluxed for 1.5 hours. The cooled solution was washed with10% sodium bicarbonate, and the organic phase was concentrated atreduced pressure. The residue was purified by chromatography on silicagel, eluting with 30% ethyl acetate-hexane, producing the oxazolidinoneas a clear oil (1.01 g, 80%). ¹H NMR (300 MHz, CDCl₃) 7.39-7.34 (m, 5H),5.27 (s, 2H), 5.16 (s, 2H), 1.57 (s, 6H). ¹³C NMR (75 MHz, CDCl₃)175.47, 135.51, 128.62, 128.44, 128.12, 76.127, 67.29, 56.97, 22.54.Anal. Calcd for C₁₂H₁₅O₄N C, 62.64; H, 6.07; N, 5.62. Found C, 62.74; H,6.07, N, 5.67%.

Example 11 Preparation of N-Benzyloxycarbonyl-N-methyl α-AminoIsobutyric acid

Z-AIB—OH (1.17 g, 4.7 mmol) was dissolved in chloroform* (11.2 mL) andtrifluoroacetic acid (11.2 mL) and the mixture was heated to 50° C. withstirring. To this mixture was added triethylsilane (1.65 mL, 15 mmol)and stirred at 50° C. for 16 hours. The solution was concentrated atreduced pressure, diluted with chloroform* (25 mL) and then concentratedagain at reduced pressure. Purification of the residue was achieved bychromatography on silica, eluting with 5% methanol-chloroform, producinga white solid (800 mg, 68%). *Chloroform was washed with water and driedover magnesium sulphate to remove the ethanol stabiliser. ¹H NMR (300MHz, CDCl₃), (s, 1H), (m, 5H), (s, 2H), (s, 3H), (s, 6H). ¹³C NMR (75MHz, CDCl₃) 197.94, 197.54, 156.13, 136.10, 128.36, 127.93, 127.87,67.55, 60.74, 29.63, 23.62.

Example 12 Preparation of(S)-3-Benzyloxycarbonyl-4-(3-hydroxypropyl)-oxazolidin-5-one

Z-glutamic acid (500 mg) was dissolved in dry THF (5 mL), under anitrogen atmosphere with stirring. To the solution was added 1M THF.BH₃complex (3.4 mL), and the mixture was left to stir overnight. Thesolution was concentrated in vacuo and the residue was left to stand(the boric acid precipitates), dissolved in dichloromethane, filteredand further purified by chromatography on silica, eluting with 40% ethylacetate-hexane to produce a clear oil (150 mg, 31%). ¹H NMR (300 MHz,CDCl₃) 7.58-7.37 (m, 5H), 5.50 (br s, 1H), 5.20-5.07 (m, 2H), 2.46 (brs, 2H), 2.33-2.12 (m, 2H). ¹³C NMR (75 MHz, CDCl₃) 171.74, 152.26,135.00, 128.24, 128.06, 127.95, 77.48, 67.56, 63.67, 32.92, 18.44.

Example 13 Preparation of(S)-3-Benzyloxycarbonyl-4-vinyl-oxazolidin-5-one

Z-Allylglycine-OH (500 mg) in toluene (20 mL), in a 100 mL roundbottomed flask, was fitted with a water condenser. To this mixture wasadded paraformaldehyde (0.2 g) and a catalytic amount of camphorsulfonicacid and the mixture was refluxed for 2 hours. The solution wasconcentrated in vacuo, and purified on silica gel, eluting with 30%ethyl acetate-hexane to produce a clear oil (410 mg). ¹H NMR (300 MHz,CDCl₃) 7.40-7.36 (m, 5H), 5.24-5.09 (m, 3H), 4.43 (br s, 1H), 2.60 (brs, 2H). ¹³C (75 MHz, CDCl₃) 173.36, 152.15, 135.15, 128.25, 128.06,127.50, 120.62, 77.79, 67.49, 54.63.

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It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A compound of formula I or II:

in which R¹ is an N-protecting group or a peptide; R² is CHCH₃OAc orCHR⁵R⁶ in which R⁵ is hydrogen and R⁶ is OAc, CONH₂, SBn,

CO2R7 or CH2CO2R7 in which R7 is a carboxyl protecting group; and R3 isCHCH3OAc,

or CHR5R6 in which R5 is as defined above and R6 is OAc, SBn, CONHTrt,

CO₂R⁷, CHCO₂R⁷, CH₂CH₃ or CH═CH₂ in which R⁷ is as defined above, R⁸ isa histidine protecting group and R⁹ is a phenol protecting group; R⁴ ishydrogen or R⁴ is methyl when R³ is OAc; R³ together with R⁴ formscyclopentyl; or R² and R³ independently represent optionally protectedamino acid side chains selected from:

salts, hydrates, solvates, derivatives, tautomers and/or isomersthereof.
 2. A compound according to claim 1, which is selected from:

in which R¹ is as defined in claim
 1. 3. A process for preparing thecompound of formula I as defined in claim 1 which comprises reductivecleavage of the compound of formula II as defined in claim
 1. 4. Aprocess according to claim 3 in which the reductive cleavage employstrifluoroacetic acid (TFA) as the acid and triethylsilane (Et₃SiH) asthe reductant.
 5. A process for preparing the compound of formula I orII as defined in claim 1 when R¹ is an N-protecting group or a peptide;R² is CHCH₃OAc or CHR⁵R⁶ in which R⁵ is hydrogen and R⁶ is OAc, CONH₂,SBn,

CO₂R⁷ or CH₂CO₂R⁷ in which R⁷ is a carboxyl protecting group; and R³ isCHCH₃OAc,

or CHR⁵R⁶ in which R⁵ is as defined above and R⁶ is OAc, SBn, CONHTrt,

CO₂R⁷ CHCO₂R⁷, CH₂CH₃ or CH═CH₂ in which R⁷ is as defined above, R⁸ is ahistidine protecting group and R⁹ is a phenol protecting group; R⁴ ishydrogen or R⁴ is methyl when R³ is OAc; R³ together with R⁴ formscyclopentyl; which comprises the steps of: (a) converting a compound offormula III

in which R² _(a) is CHOHMe or CHR⁵R⁶ _(a) in which R is as defined aboveand R⁶ _(a) is OH, SH, CONH₂,

in which R⁸ is as defined above,

CO₂H or CH₂CONH₂ or salts thereof into a compound of formula IV

in which R¹ _(b) is an N-protecting group; R² _(b) is CHOAcMe or CHR⁵R⁶_(b) in which R⁵ is as defined above and R⁶ _(b) is OAc, SBn, SMe,CONHR¹ _(b) in which R¹ _(b) is as defined above,

CO²H or CH₂CO₂H; (b) oxazolidination of the compound of formula IV toform the compound of formula II; and (c) reductive cleavage of thecompound of formula II to form the compound of formula I.
 6. A processaccording to claim 5, in which the conversion step (a) results in theprotection of the amino group on the compound of formula III to producethe compound of formula IV.
 7. A process according to claim 5, in whichthe oxazolidination step (b) uses a formaldehyde source in an organicsolvent.
 8. A process according to claim 7, in which the formaldehydesource is paraformaldehyde and paratoluenesulphonic acid (TsOH).
 9. Aprocess according to claim 7, in which the organic solvent is benzene ortoluene.
 10. Use of the compound of formula I or II defined in claim 1in the synthesis of peptides.
 11. A peptide which includes the compoundof formula I or II as defined in claim
 1. 12. A peptide according toclaim 11, which is a dipeptide.
 13. A peptide according to claim 12, inwhich the dipeptide is of the formula V

in which R¹ and R² are as defined in claim 1 or claim 2, R′ is anoptionally protected amino acid side chain and R is H or acarboxyl-protecting group.
 14. A kit for use in synthesising peptideswhich comprises (a) at least one compound of formula I or formula II asdefined in claim 1; and (b) optionally at least one other N-methyl aminoacid, its precursor oxazolidinones, an optionally substituted aminoacid, or protected forms thereof, said compounds, N-methyl amino acids,oxazolidinones and/or amino acids being held separately.
 15. A kit foruse in synthesising peptides which comprises (a) peptide as defined inclaim 11; and (b) optionally at least one other N-methyl amino acid, itsprecursor oxazolidinones, an optionally substituted amino acid, orprotected forms thereof, said compounds, N-methyl amino acids,oxazolidinones and/or amino acids being held separately.